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A prefix combined with a number to indicate a protein of a specific molecular size (mass). Often given initially if no specific "name" has been given to the protein, see examples in the following entries.


a cell cycle protein in mammals that suppresses G1-Cdk activity in G1 and is frequently inactivated in cancer. Belongs to a class of Cdk inhibitory proteins. Nomenclature is p for protein and 16 is molecular size (16,000 daltons).
(More? Mitosis | OMIM Entry for p16)


(tp53) Nomenclature is t for tumor, p for protein and 53 is molecular size (53,000 daltons). A cell cycle related transcription factor that promotes transcription of genes that induce cell cycle arrest or apoptosis in response to DNA damage or other cell stresses. This tumor suppressor gene is mutated in about half of all human cancers. Shown to also have a role in maternal reproduction in mice through transcriptional regulation of leukemia inhibitory factor (LIF), a cytokine required for blastocyst implantation and also required in human fertility.
(More? Mitosis | OMIM Entry for p53 | PMID19470478 | Review - PMID19776744)


a p53 related transcription factor and is essential for the development of stratified epithelia. p63 acts as a "master regulator" controlling stratified epithelial development program, through induction of downstream signals important for epithelial function. Nomenclature is p for protein and 63 for the molecular size (63,000 daltons).
(More? Reference: R.A. Ihrie etal., Perp Is a p63-Regulated Gene Essential for Epithelial Integrity See also : OMIM Entry for p63)

pachytene stage

(pachytene phase, pachynema; Greek, pachytene = "thick threads") A meiotic cell division stage seen during prophase I. Prophase I is further divided into 5 stages based upon changes associated with the synaptonemal complex structure that forms between two pairs of homologous chromosomes. In pachytene, the synaptonemal complex is complete and can be stable for some time. Autosomal non-sister chromatids of homologous chromosomes can now extensively exchange segments in regions of homology.
Prophase I stages: leptotene - zygotene - pachytene - diplotene - diakinesis
(More? Cell Division - Meiosis | Oocyte Development | Spermatozoa Development | Week 1 | MBoC)


A a complex neurological response to a range of stressful or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage."

palatal rugae

(rugae palatinae) The transvere ridges forming on the secondary palate which are also sequentially added during development as the palate grows. Their number and arrangement on the hard palate of mammals is species specific. Along with the teeth and the tongue these ridges function in the process of mastication.
(More? Palate Development | Head Development | Medline Plus - Cleft Lip and Palate)

palatal shelf

Paired embryonic structures that develop laterally beside the tongue, elevate and form the palate.
(More? Palate Development | Head Development | Lecture - Head Development | Medline Plus - Cleft Lip and Palate)


The roof of the mouth (oral cavity) a structure which separates the oral from the nasal cavity. Develops as two lateral palatal shelves which grow and fuse in the midline. Initally a primary palate forms with fusion of the maxillary processes with the nasal processes in early face formation. Later the secondary palate forms the anterior hard palate which will ossify and separate the oral and nasal cavities. The posterior part of the palate is called the soft palate (velum, muscular palate) and contains no bone. Abnormalities of palatal shelf fusion can lead to cleft palate.
(More? Palate Development | Head | Head Abnormalities | Medline Plus - Cleft Lip and Palate)


The process of palate formation, divided into primary and secondary palate development.
(More? Palate Development | Head | Head Abnormalities | Medline Plus - Cleft Lip and Palate)


Cell biology term for the addition of palmitate, a fatty acid, to a protein. This post-translational lipid modification requires the enzymatic (palmitoyl acyltransferase) addition of palmitate to specific protein cysteine amino acid residues through a thioester linkage. The addition can be either either permanent or transient and this dynamic can in term regulate protein function and be involved in signalling.


(protein associated with Lin7; Mpp5) An epithelial protein localized at a subapical membrane region (SAR) above intercellular adherens junctions. Also identified in cortical neuron development for cell survival through interactions with mTOR signaling.
(More? Neural System Development | PMID10753959)


The gastrointestinal tract associated organ with both exocrine (pH change and digestive enzyme secretion) and endocrine (hormone secretion) functions. In humans, the pancreas develops at the foregut/midgut junction (the septum transversum) and initially form connected to the gastrointestinal tract as two pancreatic buds (dorsal and ventral endoderm) which later fuse to form a single organ. The pancreas exocrine function (alkylate acidic stomach contents and amylase protein digestion) begins mainly fter birth. The endocrine function (alpha cell - glucagon, delta cell - somatostatin, beta cell - insulin) can be measured from 10 to 15 weeks onward.
(More? Gastrointestinal Tract - Pancreas | Endocrine Development - Pancreas)

pancreatitis-associated protein -III

(PAP-III) a macrophage chemoattractant, implicated in nerve regeneration as it has been shown to be induced and released from injured nerves.
(More? Journal of Neuroscience Paper)

Paneth cell

A differentiated cell type cell within the gastrointestinal system epithelium. Paneth cells are located at the bottom of intestinal crypts (crypts of Lieberkühn) in the small intestine. Their role is to control of infections by release of several antibacterial substances, including lysozyme. Paneth cell differentiation and positioning is apparently controlled by the developmental signaling protein Wnt.
(More? Gastrointestinal Tract | Molecular Development | Wnt)

Panton-Valentine leucocidin toxin

(PVL) A bacterial toxin produced by about 2% of Staphylococcus aureus (S aureus) bacteria, occurs very rarely and can be fatal in neonates.
(More? Abnormal Development - Bacterial Infection)

Papanicolaou stain

(Papanicolaou's stain, Pap stain) Histological staining technique used to differentiate cells in smear preparations of various bodily secretions including gynecological female smears (Pap smears) and male seminal fluid. Developed by George Papanikolaou (1883 - 1962) and consisting of 5 dyes in 3 solutions generating a multichromatic stain of cells.
(More? Menstrual Cycle - Histology | Menstrual Cycle)


An acronym for pregnancy-associated plasma protein-A. Protein produced by both embryo and placenta during pregnancy. Maternal serum concentrations are related to subsequent fetal growth and it is thought to have several different functions, including preventing recognition of the fetus by the maternal immune system.
(More? pregnancy-associated plasma protein-A)

paramesonephric duct

(Müllerian duct) (Greek, para = "beside") The paired ducts that lie beside the mesonephric ducts, that will differentiate in the female embryo to form the female internal genital tract (uterine tubes, uterus, upper vaginal canal). Paramesonephric duct differentiation in females requires Wnt (Wnt7a, Wnt5a) signaling through the intracellular β-catenin pathway.
(More? Uterus Development | Genital - Female Development | Genital System Development)

paranasal sinuses

Air-filled cavities surrounding the nasal cavity and open into it, which combine in function to: reduce skull weight, produce mucus, and act as resonating chambers affecting voice quality. Located within in the frontal, maxilae, ethmoid, and sphenoid bones with the same name as the bones in which they are located.
(More? Respiratory | Image - Nasal Cavities)


(Latin, sagitta = "arrow") The sagittal plane (ventro-dorsal) sections away from the midline (median plane or mid-sagittal) taken through the body.

parathyroid hormone

(PTH, parathormone or parathyrin) A polypeptide (84 amino acids) hormone secreted by the parathyroid gland, which increases the concentration of calcium ions in the blood. Its actions oppose the hormone calcitonin from the thyroid gland parafollicular cells (C cells), which decrease calcium. Acts through the parathyroid hormone receptor located mainly in bone, kidney and gastrointestinal tract. Hormone dual role is to: stimulate osteoclasts in bone to degrade bone matrix releasing calcium; increase gastrointestinal tract absorption of calcium.
(More? Parathyroid | Thyroid)

parathyroid hormone-related protein

(PTHrP) A protein named for its evolutionary and structural relationship to parathyroid hormone (PTH). A protein hormone produced by many fetal tissues and with a number of different functions including endochondral bone development, tooth development, and mammary gland epithelial-mesenchymal developmental interaction. Also suggested to have an autocrine role in lung development.
(More? Parathyroid | Respiratory Bone)

paratubal musculature

The muscles lying beside the auditory tube (Eustachian) consisting of the tensor veli palatini (TVP) and tensor tympani muscles.
(More? Lecture - Hearing | Lecture - Face and Ear Development)

paraxial mesoderm

In early development, the two lateral strips of mesoderm lying beside the axial mesoderm (notochord). This mesoderm at the body level will segment into somites, at the head level it remains unsegmented.
(More? Musculoskeletal System Development | Week 3)


(Greek, enkeim = "to pour in") Term used to describe the cells forming the functional cells of an organ or tissue. These cells carry out the function of the organ at a cellular level. Within the organ, the other population of cells, [S.htm#stroma stroma], are structural cells, connective tissue, the non-cellular extracellular matrix.

parietal pleura

Serous membrane which forms the outer lining of pleural cavity. mesoderm of the thoracic cavity body wall and derived from epithelia of pericardioperitoneal canals from intraembryonic coelom. The inner pleural layer, visceral pleura, is splanchnic mesoderm in origin.
(More? Respiratory)


Clinical term referring to the number of a woman’s previous pregnancies that resulted in a birth.
(More? Birth | Birth Statistics)

Parkinson's disease

(paralysis agitans, shaking palsy) A postnatal neurological disease of the central nervous system, typically in ageing and more common in men than in women and has a genetic and/or environmental cause. The neurons affected are those in the brain that control muscle movement and have dopamine as their neurotransmitter. Named after James Parkinson (1730-1813), an English physician who made the detailed description of the disease in "An Essay on the Shaking Palsy" (1817). Identified mendelian forms of parkinsonism: autosomal dominant forms alpha-synuclein (SNCA) and LRRK2; autosomal recessive forms parkin (PARK2), PINK1, DJ-1(PARK7) and ATP13A2.
(More? Neural | Neural Abnormalities | Medline Plus - Parkinson's Disease | OMIM 168600 | PMID2666579)


(Latin, pars = part of) Anatomical term describing something as part of an organ or tissue.


(Greek, parthenos = virgin, genesis = birth) The development of an unfertilized egg (no sperm). Other than mammals, many different species (plants, insects, reptiles) can develop from unfertilized eggs. An embryo so formed without sperm contribution. Blocking of parthenogenesis in mammals appears to be related to genomic imprinting. Abnormal parthenogenic processes can occur in mammals, and more recently a parthenogenic mouse has been made in the laboratory.
(More? Week 1)

partial androgen insensitivity

A genetic disorder causing ambiguous genitalia in 46,XY individuals. This X-linked disease is due to an absence or deficiency of androgen receptors in the androgen target tissues (e.g. genital skin region).
(More? Genital - Abnormalities)


(Latin, parturitio = childbirth) Clinical term used to describe the entire process of childbirth.
(More? Birth)


(Latin, parvo = poor) A family of viruses that infect many species. Human parvovirus B19 (a single-strand 5,594 nucleotide DNA Class II virus) infection is also called "fifth disease" and occurs mainly in children. Animal parvoviruses do not infect humans.
(More? Parvovirus)

passive immunity

(maternal passive immunity) Term used to describe the transfer of antibodies from one person to another, this can occur artificially or by maternal transfer (maternal passive immunity) to the fetus and neonate.
(More? Placenta Development | Normal Development - Milk)

patched receptor

A membrane protein that binds the signaling factor sonic hedgehog during development.
(More? Developmental Signals - Sonic hedgehog)

patent ductus arteriosus

(PDA) Term describing a cardiac abnormality due to due to partial or complete failure of closure of the ductus arteriosus, an embryonic blood shunt bypassing the pulmonary system. Antenatal diagnosis of patent arterial duct is not possible as it is a normal structure during antenatal life and there are no accurate antenatal genetic tests. This occurs commonly in preterm infants and can close spontaneously (by day three in 60% of normal term neonates) the remainder are ligated simply and with little risk. The operation is always recommended even in the absence of cardiac failure and can often be deferred until early childhood.
(More? Cardiovascular System - Abnormalities | PDA Classification | PDA echocardiogram | PDA angiogram | Cardiac Tutorial | Lecture - Heart)


Term used in relation to the male father and is used genetically, biologically and legally. The term maternal relates to the female mother.
(More? Spermatozoa Development)


The name derived from Drosophila gene "paired" (prd) with a box (homeodomain) domain. A transcription factor of the helix-turn-helix structural family, DNA binding, and activating gene expression. In human, there are nine member proteins from Pax1 to Pax9. Pax6 has been identified as regulating development of the central nervous system, eyes, nose, pancreas and pituitary gland.
(More? Pax | Molecular Development)


Acronym for polychlorinated biphenyls, a class of chemicals linked to developmental defects.
(More? Abnormal Development - Polychlorinated Biphenyls)


(2,2,4,4,5,50-hexachlorobiphenyl) A form of polychlorinated biphenyls, a class of chemicals linked to developmental defects.
(More? Abnormal Development - Polychlorinated Biphenyls)

Peabody Developmental Motor Scale II

(PDMS-2) An early postnatal neurological assessment scale used in screening and diagnosis of neural development.
(More? Neural | Neural System - Postnatal)


(parenchyma) Histological term used to describe the functional cells of an organ, tissue or structure. The term is often paired with stromal (stroma), which describes the supportive cells within an organ, tissue or structure.


(Latin, pediculus = small foot) Region of the vertebra, forming the vertebral arch portion between the transverse process and the vertebral body.


(resting secretory cell) Histological term for the non-ciliated secretory epithelial cells located within the uterus.
(More? Uterus Development)

pelvic inflammatory disease

(PID, endometritis, salpingitis) Pathology due to an ascending infection of the upper female genital tract, usually a sexually transmitted disease (STD), mainly by Gram-negative bacteria. May also occur following childbirth, miscarriage, or a surgical procedure.
(More? Uterus Development | Ectopic Implantation)


(Latin, pelvis = basin) Anatomical term referring to the basin-shaped skeleton and region formed by the axial and appendicular skeleton bones: innominate (lateral), pubis (anterior) and the sacrum and coccyx (posterior). The body space it encloses is the pelvic cavity. Term is also used to refer to the basin-shaped drainage region of the kidney, the renal pelvis.
(More? Musculoskeletal System Development | Renal System Development)

Pendred syndrome

(PDS) Clinical term describing an autosomal recessive disorder characterized by sensorineural hearing impairment and variable degree of goitrous enlargement of the thyroid gland with a partial defect in iodine organification. Pendred syndrome is caused by mutation in the anion transporter known as Solute Carrier family 26 member 4 (SLC26A4) gene.
(More? Endocrine - Thyroid Development | OMIM - Pendred | OMIM - SLC26A4)

Pentalogy of Cantrell

A developmental abnormality of the anterior diaphragm, diaphragmatic pericardium, abdominal wall, cardiovascular and lower sternum.
(More? Diaphragm Abnormalities | Musculoskeletal Abnormalities)

pericardioperitoneal canal

(pleural canal) In early development, the intraembryonic coelom region that joins the pericardial cavity to the peritoneal cavity, occurring as two separate spaces either side of the the pericardial cavity, this region later develops into the paired pleural cavities.
(More? Coelomic Cavity Development)


Layer that covers the heart, formed by 3 layers consisting of a fibrous pericardium and a double layered serous pericardium (parietal layer and visceral epicardium layer). The other two cardiac layers are the myocardium and endocardium. The heart embryonic origin is from the cariogenic region of prechordal splanchnic mesoderm.
(More? Cardiovascular System Development | Cardiac Embryology tutorial)


The undifferentiated mesenchymal cells which surround the growth plate cartilage and regulate chondrocyte differentiation. Involved with endochondral bone formation process.
(More? Bone Development)


The smooth muscle-like cell which covers over blood vessel endothelial cells.
(More? Cardiovascular System Development | Cardiovascular System - Lymphatic Development)


The early postnatal period relating to the birth, statistically it includes the period up to 7 days after birth.
(More? Birth)

perinatal mortality ratio

A statisical term which refers to the sum of late fetal deaths plus infant deaths within 7 days of birth divided by the number of live births, per 1,000 live births.
(More? Birth | Statistics | Australian Statistics)

perinuclear theca

(PT)The region located in the sperm head perinuclear region, containing a cytoskeletal element to maintain the shape of the sperm head and functional molecules leading to oocyte activation during fertilization. The perinuclear theca surrounds the nucleus and is also divided into two major regions: the subacrosomal layer and the post-acrosomal sheath (PAS) at the post-acrosomal region (PAR).
(More? Spermatozoa Development | Testis Development | Fertilization)

periodontal ligament

(PDL) Set of specialized connective tissue fibres that attach a tooth to the alveolar bone. The fibres both attach the tooth and also function to withstand compressive forces that occur during chewing. The collagen fiber bundles within the ligament are called "Sharpey’s fibres". Cementum, from investing layer of the dental follicle, is a contiguous layer with the periodontal ligament on one surface and firmly adherent to dentine on the other surface.
(More? Tooth Development)


fibrous membrane covering bone except on the surface of joints which are covered in articular cartilage.
(More? Musculoskeletal System Development | Bone Development)

perisylvian cortex

The region of the cortex (frontal and posterior) involved in the human cognitive specializations which include language. The perisylvian region for language is also asymmetrically represented in the area of left hemisphere. The posterior perisylvian superior temporal gyrus (STG)
(More? Neural Development | PMID17978184)

pericardial cavity

The anatomical body cavity in which the heart lies. The pericardial cavity forms in the lateral plate mesoderm above the buccopharyngeal membrane, as part of the early intraembryonic coelom. This cavity is initially continuous with the two early pleural cavities. Note the single intraembryonic coelom forms all three major body cavities: pericardial cavity, pleural cavity, peritoneal cavity.
Intraembryonic Coeloms: pericardial cavity - pleural cavity - peritoneal cavity
(More? Coelomic Cavity Development | Cardiovascular System Development)

peritoneal cavity

The anatomical body cavity in which the lower body organs lie: intestines, liver, bladder, uterus, ovary. The peritoneal cavity forms initially from two separate regions of the early intraembryonic coelom (formed in the lateral plate mesoderm), which with embryo folding, fuse to form a single cavity. Note the initial single intraembryonic coelom will form all three major body cavities: pericardial cavity, pleural cavity and peritoneal cavity.
Intraembryonic Coeloms: pericardial cavity - pleural cavity - peritoneal cavity
(More? Coelomic Cavity Development | Gastrointestinal Tract Development)

peritubular cells

(peritubular myoid cells) stromal cells (mesenchymal) located in the male gonad (testis) seminiferous tubule lying outside the basal membrane in the lamina propria surrounding seminiferous tubules that regulate Sertoli cell function and contractility of seminiferous tubules. This term is also used to describe the cells found in the kidney glomerulus.
(More? Testis Development | Spermatozoa Development)

perivitelline space

(PVS) space between the oocyte (egg) and the surrounding zona pellucida, enlarged in the region where the polar body lies. In mice this has been shown to influence the site sperm entry independent of any asymmetry of the oocyte membrane.
(More? Week 1 - Fertilization | PMID16620153)


(heparan sulfate proteoglycan 2) An extracellular matrix proteoglycan expressed in the basal lamina of the neuroepithelium.
(More? Neural System Development | Extracellular Matrix Lecture)

Perlman syndrome

(renal hamartomas, nephroblastomatosis, fetal gigantism) A rare overgrowth syndrome characterized by polyhydramnios with neonatal macrosomia, nephromegaly, distinctive facial appearance, renal dysplasia, nephro-blastomatosis, and predisposition to Wilms tumor. Molecular basis of Perlman syndrome is unknown and there is a high neonatal lethality and an intellectual disability in those survivors.
(More? PMID18780370)

permanent neonatal diabetes

(PNDM) One of the two main forms of neonatal diabetes mellitus diagnosed within the first 6 months of life. This form of diabetes has been shown to result from mutations in the preproinsulin (INS) gene. The other clinical subtype is the remitting and frequently relapsing transient neonatal diabetes (TNDM).
(More? Endocrine - Pancreas Development | Abnormal Development - Maternal Diabetes)

Persistent Pulmonary Hypertension of the Newborn

(PPHN) A serious newborn condition due to due to the failure of closure one of the prenatal circulatory shunts, the ductus arteriosus. Occurs in about 1-2 newborns per 1000 live births and results in hypoxemia.
(More? Respiratory Development - Birth)

persistent right umbilical vein

(PRUV) A placental cord abnormality associated with fetal abnormalities and poor neonatal prognosis. The estimated incidence of persistent right umbilical vein in a low-risk population is 1 : 526. PMID: 12047534
(More? Placenta Development | PMID12047534)

persistent organic pollutants

(POPs) These are organic chemical substances with a combination of physical and chemical properties that once released into the environment: remain intact for exceptionally long periods of time (many years); become widely distributed throughout the environment as a result of natural processes involving soil, water and, most notably, air; accumulate in the fatty tissue of living organisms including humans, and are found at higher concentrations at higher levels in the food chain; and are toxic to both humans and wildlife.
(More? Abnormal Development - Chemicals | Abnormal Development - Environmental | Stockholm Convention on Persistent Organic Pollutants | What are POPs?)

persistent truncus arteriosus

A developmental heart (cardiac) abnormality (1 to 2% of all heart defects) , where the truncus arteriosus fails to divide into aorta and pulmonary artery. The defect also involves a ventricular septal defect and can be associated with DiGeorge syndrome or velocardiofacial syndrome.
(More? Cardiovascular System - Abnormalities | Cardiovascular System Development)

Peyer's patches

The gut-associated lymphoid tissue (GALT) located in the small intestine (usually the ileum) and appendix (lying between the lamina propria to the submucosa). These immune system structures are the first line of protection against ingested infectious agents. Peyer’s patches consist of a dome area, B-cell follicles and interfollicular T-cell areas.
(More? Gastrointestinal Tract)


An acronym for Primordial Germ Cell, the embryonic cell that will form eventually either oocyte (egg) or spermatozoa (sperm) cell.
(More? Primordial Germ Cell Development | Primordial germ cell migration | Genital)


A molecular acronym for Peroxisome proliferator-activated receptor gamma coactivator 1-alpha a transcriptional coactivator that regulates many different pathways linked to energy homeostasis in many tissues through regulation of mitochondrial biogenesis. Other tissue-specific functions include skeletal muscle fibre type switching, brown adipose tissue (BAT) thermogenesis, and liver gluconeogenesis and fatty acid oxidation. In the liver, gluconeogenesis activity following feeding is decreased by phosphorylation of PGC-1α.
(More? OMIM 604517)


Acronym for Pre-implantation Genetic Diagnosis, a screening procedure for embryos produced through in vitro fertilization (IVF) for genetic diseases that would generate developmental abnormalities or serious postnatal diseases.
(More? Week 1 | In Vitro Fertilization)

pharyngeal arch

(branchial arch, Greek, branchial = gill) These are a series of externally visible anterior tissue bands lying under the early brain and above the heart that give rise to the structures of the face, head and neck. In humans, five arches form (1,2,3,4 and 6) but only four are externally visible on the embryo. Each arch has initially identical structures: an internal endodermal pouch, a mesenchymal (mesoderm and neural crest) core, a membrane (endoderm and ectoderm) and external cleft (ectoderm). Each arch mesenchymal core also contains similar components: blood vessel, nerve, muscular, cartilage. Each arch though initially formed from similar components will differentiate to form different head and neck structures.
(More? Head Development | Endocrine | Neural Crest)

pharyngeal arch artery

Each early developing pharyngeal arch contains a lateral pair of arteries arising from the aortic sac, above the heart, and running into the dorsal aorta. later in development these arch arteries are extensively remodelled to form specific components of the vascular system. Pharyngeal Arch 1 arteries are mainly lost and forms part of maxillary artery. Pharyngeal Arch 2 arteries remains to form the stapedial arteries. Pharyngeal Arch 3 arteries forms the common carotid arteries, internal carotid arteries in the neck. Pharyngeal Arch 4 arteries will form part of aortic arch (left arch artery) and part right subclavian artery (right arch artery) Pharyngeal Arch 6 arteries form part of left pulmonary artery (left arch artery) and part of right pulmonary artery (right arch artery).
(More? Head Development | Cardiovascular)

pharyngeal arch cartilage

Each early developing pharyngeal arch contains a horseshoe shaped band of cartilage that acts as a template and contributes to the development of head and neck bony and cartilagenous features, including the middle ear bones. Pharyngeal Arch 1 cartilage (Meckel's cartilage) dorsal ends form malleus and incus midpart forms ligaments (ant. malleus, sphenomandibular) ventral part forms mandible template. Pharyngeal Arch 2 cartilage (Reichert's cartilage) dorsal ends form stapes and Temporal bone styloid process, ventral part ossifies to form hyoid bone components, lesser cornu and superior body. Pharyngeal Arch 3 cartilage forms hyoid components, greater cornu and inferior part of hyoid. Pharyngeal Arch 4 and 6 cartilage forms laryngeal cartilages except epiglottis (from hypobranchial eminence).
(More? Head Development | Neural Crest | Middle Ear)

pharyngeal arch nerve

Each early developing pharyngeal arch contains the developing cranial nerves, as a pair, within the arch mesenchyme. Each cranial nerve is numbered (roman numeral) in rostrocaudal sequence and also has a specific name. The cranial nerve within each arch often relates to the other structures formed from taht arch. Pharyngeal Arch 1 contains the trigeminal nerve (CN V, cranial nerve 5). Pharyngeal Arch 2 contains the facial nerve (CN VII, cranial nerve 7). Pharyngeal Arch 3 contains the glossopharyngeal nerve (CN IX, cranial nerve 9) Pharyngeal Arch 4 and 6 contains the Vagus (CN X cranial nerve 10), forming the adult superior laryngeal and recurrent laryngeal branches.
(More? Head Development | Neural | Neural Crest)

pharyngeal arch pouch

An out-pocketing of the endoderm lined pharynx occurring between each developing pharyngeal arch. Each of the pharyngeal arch pouches contributes different components of the head and neck, either cavities or endocrine tissues. Pharyngeal Arch 1 pouch elongates to form tubotympanic recess tympanic cavity, mastoid antrum and auditory tube (Eustachian tube). Pharyngeal Arch 2 pouch forms the tonsillar sinus and is later mostly oblierated by palatine tonsil. Pharyngeal Arch 3 pouch forms the inferior parathyroid and thymus. Pharyngeal Arch 4 pouch forms the superior parathyroid, parafollicular cells of Thyroid.
(More? Middle Ear | Thyroid | Parathyroid | Thymus‎ | Endocrine | Head Development

pharyngotympanic tube

(auditory tube, eustachian tube, otopharyngeal tube) A narrow canal connecting the middle ear space to the back of the oral cavity. The tube allows ventilation, protection and clearance for the middle ear cavity. Ventilation is the pressure equalization in the middle ear. Clearance is to allow fluid drainage from the middle ear. Embryonic origin is from the first pharyngeal pouch. In development, the canal is initially both horizontal, short and very narrow leading to poor drainage and easy blockage.
(More? Middle Ear | Hearing | Hearing Abnormalities)


(throat) In the embryo the uppermost end of the combined gastrointestinal and respiratory tract beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches. Also used as a respiratory term describing the initial segment of the upper respiratory tract divided anatomically into three regions: nasopharynx, oropharynx, and laryngopharynx (hypopharynx). Anatomically extends from the base of the skull to the level of the sixth cervical vertebra.
(More? Head Development | Gastrointestinal Tract Development | Respiratory System Development)


Term used to describe the observable characteristics of an organism and is related to the expressed genotype.


(PKU) A disease resulting from an inability to break down phenylalanine, an amino acid. Causes mental and motor retardation, microcephaly, poor growth rate and seizures. Can be tested in newborn screening.
(More? Neonatal Diagnosis)


A secreted chemical in sweat or urine that causes specific physiological response in other members of the same species.
(More? Mouse Estrous Cycle)

Philadelphia chromosome

(Ph, Philadelphia translocation) Genetic term referring to a chromosomal abnormality resulting from a reciprocal translocation between chromosome 9 and 22 (t(9;22)(q34;q11)). This chromosomal abnormality is associated with the disease chronic myelogenous leukemia (CML).
(More? Philadelphia chromosome | Abnormal Development - Genetic | Cell Division - Meiosis | Cell Division - Mitosis | Week 1 | PMID17671636)

Philadelphia chromosome-negative chronic myeloproliferative neoplasm

Term used to describe the three myeloproliferative disorders: polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) not associated with the Philadelphia chromosome translocation.

Philadelphia translocation

(Philadelphia chromosome) See Philadelphia chromosome.


(infranasal depression, Greek, philtron = "to love" or "to kiss") Anatomically the surface midline vertical groove in the upper lip. Embryonically formed by the fusion of the frontonasal prominence (FNP) with the two maxillary processes of the first pharyngeal arch. Cleft palate (primary palate) occurs if these three regions fail to fuse during development. Fetal alcohol syndrome is also indicated by flatness and extension of this upper lip region.
(More? Head Development | Palate Development | Lecture - Head Development | Medline Plus - Cleft Lip and Palate)


(Greek, phoco = seal, melia = limb) a flipper-like appendage attached to the trunk, as described in the original classical classification of limb deficiencies. Seen in genetic and environmentally derived abnormalities, for example in association with thalidomide.
(More? Musculoskeletal System - Limb Abnormalities)


See protein phosphatase.


Acronym for Placental Growth Factor a vascular endothelial growth factor (VEGF) released from the placental trophoblast cells and other sources.

pia mater

A fine connective tissue covering of the central nervous system, forms innermost part of the meningial layers. Lies beneath the arachnoid mater and then tough outer dura mater layer. These three connective tissue layers have different embryonic origins: pia mater and arachnoid layers are neural crest in origin, dura is from mesoderm. The space overlying the pia mater (subarachnoid space) is filled with cerebrospinal fluid. The pia mater has close contact with the spinal cord and brain, in the brain it follows down into the sulci and fissures of the cortex. This layer also fuses with the membranous lining of the ventricles (ependyma) forming the choroid plexus.
(More? Image - Meninges cartoon | Neural System Development | Axial Skeleton)

Pierre Robin Sequence

(PRS, Pierre Robin Syndrome, Pierre Robin Malformation) A developmental abnormality (incidence about 1:8,500) associated with pharyngeal arch and pharynx that leads to a number of facial and oral abnormalities. The 3 clinical characteristics are micrognathia, glossoptosis, and cleft palate. This is more commonly described by abnormalities include: cleft soft palate (cleft palate), high-arched palate; jaw that is very small (micrognathia) or placed unusually far back in the throat (retrognathia) with a receding chin, downward displaced tongue (glossoptosis) large compared to jaw, natal teeth (teeth appearing when the baby is born). Postnatally there are also recurrent ear infections and the small opening in the roof of the mouth can causes choking. The form of palate clefting suggests an associated with the fetal (secondary) palate and not embryological (primary) palate formation.
(More? Head Development - Abnormalities | Head Development | OMIM - Pierre Robin Syndrome)

Pierre Robin Syndrome

(PRS, Pierre Robin Sequence, Pierre Robin Malformation) See Pierre Robin Sequence. The historic name given to the condition described by Pierre Robin (1926). The term syndrome is now used for a single cause (etiology) resulting in multiple anomalies, whereas a sequence is a series of anomalies caused by a cascade of events initiated by a single malformation.
(More? Head Development - Abnormalities | Head Development | OMIM - Pierre Robin Syndrome)

pineal gland

The central nervous system endocrine gland named as the organ shape that resembled a pine cone. The embryonic origin is from a dorsal outpocket of diencephalon. The hormone melatonin is secreted and has circulating high levels in dark and low in day and is involved with the diurnal cycle, it also inhibits pituitary-gonad (HPG) axis.
(More? Pineal Development | Lecture - Endocrine Development | Endocrine System Development)

Pink Disease

A chronic mercury poisoning in infants and young, also called acrodynia (Greek, acrodynia = painful extremities)
(More? acrodynia | Heavy Metals | Michigan Health MercurySpillFactsheet_85689_7.pdf)


(uterodome) Cellular feature seen on the apical uterine epithelium surface, the presence of these structures is thought to be a marker for endometrial receptivity. These transient microprotrusions inter-digitate with microvilli on the apical syncytiotrophoblast surface of the blastocyst during adplantation and implantation process. It has been suggested that their role is not primarily pinocytotic, hence the alternate suggested name "uterodomes" based upon their appearance when imaged by electron microscopy.
(More? Week 2 | Implantation | Blastocyst | Reproductive Cycles)


An acronym for Pitrilysin metallopeptidase 1, a zinc metalloendopeptidase expressed in many embryonic tissues patterned by sonic hedgehog and also in Pax3 expressing myoblast progenitors.
SHH tissues- limbs, face, cortex, hippocampus, cerebellum, tectum, sub-mandibular gland, lung, genital tubercle, hair follicles, tooth enamel knot
myoblast progenitors - limb, dermomyotome, face and trunk developing muscles
(More? PMID19877269)

pituitary gland

(hypophysis) An endocrine gland historically called the "pituitary" as it was thought to produce mucous that discharged through the nose. The function of the hypophysis is as an endocrine gland linking the brain hypothalamus to peripheral endocrine organs and systems of the body through several specific hormones. These pathways are often described as the HPA (hypothalamus - pituitary - adrenal) and HPG (hypothalamus - pituitary - gonad) axes. The developmental origin of the hypophysis is also unique, epithelial origins from neural ectoderm (posterior, or neurohypophysis) and from surface ectoderm (anterior, or adenohypophysis).
(More? Endocrine - Pituitary Development | Lecture - Endocrine Development | Image - Fetus week 10)


An acronym for the disease phenylketonuria that can be detected by a neonatal blood test (Guthrie).


Early Placenta
(Greek, plakuos = flat cake) The developmental organ formed from maternal and fetal contributions in animals with placental development. In human, the placenta at term is a discoid shape "flat cake" shape; 20 cm diameter, 3 cm thick and weighs 500-600 gm. Placenta are classified by the number of layers between maternal and fetal blood (Haemochorial, Endotheliochorial and Epitheliochorial) and shape (Discoid, Zonary, Cotyledenary and Diffuse). The functional exchange region are the placental villi or chorionic villi. The placenta has many different functions including metabolism, transport and endocrine.
(More? Placenta Development | Lecture - Placenta Development)

placenta accreta

The abnormal placental adherence, either in whole or in part of the placenta with absence of decidua basalis, leading to retention as an after-birth to the underlying uterine wall. The incidence of placenta accreta also significantly increases in women with previous cesarean section compared to those without a prior surgical delivery.
(More? Placenta Development | Lecture - Placenta Development

placental arteries

(umbilical arteries) In placental animals, the blood vessels which develop within the placental cord carrying relatively deoxygenated blood from the embryo/fetus to the placenta. In humans, there are two placental arteries continuous with the paired internal iliac arteries (hypogastric arteries) arising off the dorsal aortas. At birth this vessel regresses and form the remnant medial umbilical ligament.
(More? Placenta Development | Lecture - Placenta Development)

placental cord

(umbilical cord) The placental cord is the structure connecting the embryo/fetus to the placenta. It is initially extra-embryonic mesoderm forming the connecting stalk within which the placental blood vessels (arteries and veins) form. In human placental cords the placental blood vessels are initially paired, later in development only a single placental vein remains with a pair of placental arteries. This structure also contains the allantois, an extension from the hindgut cloaca then urogenital sinus. Blood collected from the placental cord following delivery is a source of cord blood stem cells.
(More? Placenta Development | Lecture - Placenta Development)

placental growth factor

(PlGF) A growth factor of the vascular endothelial growth factor (VEGF) family, released from the placental trophoblast cells and other sources that stimulates blood vessel growth.
(More? Placenta Development)

placental malaria

The malarial (Plasmodium falciparum) infection of the placenta by sequestration of the infected red blood cells. This condition can be common in regions where malaria is endemic with women carrying their first pregnancy (primigravida).
(More? Malaria | Placenta - Abnormalities | Abnormal Development - Environmental | Lecture - Placenta Development | Brown University Maternal Malaria)

placenta membranacea

(PM) A rare placental abnormality where either all (diffuse placenta membranacea) or part (partial placenta membranacea) is covered by chorionic villi (placental cotyledons). Clinically the abnormality presents with vaginal bleeding, in the second or third trimester or during labor, due to an associated placenta previa.
(More? Placenta - Abnormalities | Placenta Development)

placental membranes

(chorionic membrane amniotic membrane) General term to describe the membrane bound extra-embryonic fluid-filled cavities surrounding the embryo then fetus. In humans the amniotic membrane and chorionic membrane fuse.
(More? Placenta Development | Lecture - Placenta Development | Flash movie | Quicktime movie)

placental vein

(umbilical vein) In placental animals, the blood vessels which develop within the placental cord carrying relatively oxygenated blood from the placenta to the embryo/fetus. In humans, there are initially two placental veins which fuse to form a single vein. The presence of paired veins, a persistent right umbilical vein in the placental cord can be indicative of developmental abnormalities.
(More? Placenta Development | Lecture - Placenta Development)


Term used to describe the maternal ingestion of afterbirth materials (placental membranes and amniotic fluid) that can occur following mammalian parturition (birth).


(Greek, plax = "plate" or "scale", eidos = shape or form) A surface ectoderm thickening seen in the early embryo head region either sensory or neurogenic or the adenohypophyseal placode. Each pair of placodes will contribute a different component of each sensory system (otic placode, optic placode, olfactory placode) and neurons in the trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagal (CN X) cranial ganglia. The adenohypophyseal placode will form the endocrine anterior pituitary. Note that the glial component of all cranial ganglia is neural crest in origin. The transcription factor Sox3 has been identified as an early marker of placode development.
(More? Placodes | Head Development)


One of several developmental skull deformities (scaphocephaly, oxycephaly, plagiocephaly, trigoncephaly) caused by premature fusion (synostosis) of different developing skull sutures. Plagiocephaly results from asymmetric coronal suture synostosis.
(More? Head Development | Skull Development])


Term describing the thin skin membrane (extending from the fifth digit and the posterior aspect of the forelimb, along the side of the body, to the anterior aspect of the hindlimb) forming the bat wing.
(More? Bat Development)

Plan B

A two pill high dose regimen of the oral contraceptive levonorgestrel, 90% effective in preventing pregnancy when taken within 72 hours of unprotected sex. Approved for use in some countries, but not the USA as it is considered an abortifacient not a contraceptive. US Food and Drug Administration: September 2006 - approved the emergency contraceptive Plan B for sale over the counter to women aged 18 or over. April 2009 - would accept and not appeal against a New York federal court decision that 17 year old women should be able to buy the emergency contraceptive pill Plan B without a prescription.
(More? Menstrual Cycle | Human Menstrual Cycle - Birth Control)

planar cell polarity

(PCP) A term used to describe a epithelial cellular signaling mechanism used in several developmental processes. Examples include; left-right asymmetry, initiation of neural tube closure in higher vertebrates, lung branching, and Wnt signaling.
(More? Wnt | Neural System Development)

plasma cell

A cell in the blood, the active B cell (lymphocyte) which is secreting antibody. Located in either bone marrow or peripheral lymphoid tissues, these cells have and increased cytoplasmic volume (due to increase rough endoplasmic reticulum) in comparison to the inactive (non-secreting) lymphocyte.
(More? Cardiovascular System - Blood Development | [[Cardiovascular System - Lymphatic Development])

Plasmodium falciparum

The microorganism that acts as a malarial parasite and can cause placental malaria.
(More? Malaria | [[Abnormal Development - Environmental] | Placenta - Abnormalities)

pleural cavity

Anatomical body cavity in which the lungs develop and lie. The pleural cavity forms in the lateral plate mesoderm as part of the early single intraembryonic coelom. This cavity is initially continuous with pericardial and peritoneal cavities and later becomes separated by folding (pleuropericardial fold, pleuroperitoneal membrane) and the later formation of the diaphragm. The pleural cavities form initially as two narrow canals. Note the single intraembryonic coelom forms all three major body cavities: pericardial cavity, pleural cavity, peritoneal cavity.
Intraembryonic Coeloms: pericardial cavity - pleural cavity - peritoneal cavity
(More? Coelomic Cavity Development | Respiratory System Development)

pleuropericardial fold

(pleuropericardial membrane) An early embryonic fold which restricts the communication between pleural cavity and pericardiac cavity, contains both the cardinal vein and phrenic nerve.
(More? Coelomic Cavity Development | Respiratory System Development)

pleuroperitoneal foramen

The developmental opening occurring in the intraembryonic coelom before formation of the pleuroperitoneal membrane.
(More? Coelomic Cavity Development | Respiratory System Development)

pleuroperitoneal membrane

An early embryonic membrane that forms inferiorly at the septum transversum to separate the peritoneal cavity from the pleural cavity.
(More? Coelomic Cavity Development | Respiratory System Development)


(Latin, plexus = a network) used anatomically to describe a network of interconnecting structures either nerves, blood vessels, or lymphatic vessels.


Acronym for Palate, LUng, Nasal epithelium Clone protein, related to the lipid transfer/lipopolysaccharide binding protein (LT/LBP) family. This protein is secreted by the airway conducting epithelia and acts as a surfactant that may interfere with biofilm formation by airway pathogens.
(More? Respiratory System Development | PMID20161732)


A stem cell term describing the ability of a stem cell to form (differentiate) into many different cell types. The term is often used in stem cell research instead of totipotent, which means that the stem cell can form all embryo cell types.
(More? Stem Cells)


Acronym for Pre-Menstrual Syndrome


(glomerular podocyte) Kidney epithelial cell type in the nephron (kidney functional unit) located in the glomerulus. Podocytes form the visceral layer of Bowman's capsule and are at the filtration barrier between capillary blood and the nephron tubular system and function to ultrafiltrate blood, and support glomerular capillary pressures. The differentiation of podocytes involves the formation of cellular foot processes and then the slit membrane.
(More? Renal System Development | Renal Histology| Image - podocyte development )

polar body

A small cytoplasmic exclusion body formed from the excess DNA formed during the oocyte (egg) meiotic cycle and following sperm fertilization. There are 2-3 polar bodies derived from the oocyte present in the zygote, the number is dependent upon whether polar body 1 (the first polar body formed during meiosis 1) divides during meiosis 2. This exclusion body contains the excess DNA from the reductive division (the second and third polar bodies are formed from meiosis 2 at fertilization). These polar bodies do not contribute to the genetic complement of the zygote, embryo or fetus. Recent research in some species suggest that the space formed by the peripheral polar body (between the oocyte and the zona pellucia) can influence site of spermatozoa fertilization.
(More? Oocyte Development | Cell Division - Meiosis | Week 1)

polychlorinated biphenyls

PCBs, a class of chemicals linked to developmental defects.
(More? Abnormal Development - Chemicals)

polycystin 1

(PC-1, PKD1) The protein product of the PKD1 gene, which is mutated in the majority of cases of autosomal dominant polycystic kidney disease (ADPKD). The protein is a plasma membrane receptor localized at cell-cell and cell-matrix junctions as well as cilia.
(More? OMIM - polycystic kidney disease)


(PRE/TREs) cis-regulatory DNA elements initiator elements and maintenance elements identified in drosophila development. They can activate or silence transcriptional state of associated genes for many cell generations (divisions).
(More? PubMed Review)


(hydramnios) A term referring to an excess volume of amniotic fluid, often measured by ultrasound. The opposite term for low amniotic fluid volume is oligohydramnios.

polycystic ovary syndrome

(PCOS) or Stein-Leventhal syndrome (1930s researchers) Clinical term for a metabolic syndrome leading to anovulation and with many other symptoms ( hyperandrogenism, insulin resistance). Ovarian cysts arise through incomplete follicular development or failure of ovulation (anovulation). A range of drugs can be used to induce ovulation in these women.
(More? Genital System - Abnormalities | Menstrual Cycle)


Term used to describe animals which have an estrous cycle that continues throughout the year (cattle, pigs, mouse, rat). Other types of estrous cycles include: seasonally polyestrous animals (multiple estrous cycles only during certain periods of the year) and monestrous animals (have only one estrous cycle per year).
(More? Estrous Cycle)

polysialic acid

(PSA) is a cell surface carbohydrate attached to neural cell adhesion molecule (NCAM) that regulates (inhibits) cell interactions. During development PSA has a role in promoting neural precursor cell migration and axon path-finding.
(More? Neural System Development | Image - Polysialic acid cell interactions)


(polydactylia or polydactylism) A developmental abnormality resulting in an additional fingers or toes. Occasional there is aslo a combination of polydactyly with syndactyly is known as polysyndactyly.
(More? Limb Abnormalities | Lecture - Limb Development)


(polyploid) Genetic term used to describe an abnormal number of chromosomes includes triploidy, usually due to two spermatozoa fertilizing a single oocyte. The normal genome chromosomal set (n, 2n, 3n) or complement for a species is referred to as euploidy, in humans this is diploid (2n). The other classes of numerical chromosomal abnormalities include aneuploidy and mixoploidy.
(More? Fertilization | Genetics | Abnormal Development - Genetic)


An abnormal digit (finger or toe) development resulting in a combination of polydactyly (additional digit) with syndactyly (digit fusion).
(More? Limb Abnormalities | Lecture - Limb Development)

ponderal index

(PI) - Fetal calculation based upon the ratio of body weight to length PI = [weight (in g) x 100] √∑ [length (in cm)]
(More? Fetal Development)


(Latin, pons = "bridge") A brain stem region within the central nervous system, it lies above the medulla.
(More? Pons Development | Neural System Development)

pontine flexure

In development of the central nervous system brain region, the middle early neural tube flexure between the cranial flexure and cervical flexure)). Bend is in the opposite direction to the other two flexures and also generates the thin roof of rhombencephalon and divides it into myelencephalon and metencephalon.
(More? Neural System Development)


An acronym for Persistent Organic Pollutants.
(More? Persistent Organic Pollutants | Abnormal Development - Environmental)

positive predictive value

Prenatal diagnostic test term describing the probability that a congenital anomaly is present given that the prenatal screening test is positive.
(More? Prenatal Diagnosis)

posterior coronary plexus

(plexus coronarius posterior; left coronary plexus) Neural term describing part of the peripheral nervous system (sympathetic cardiac plexus) innervating the heart (cardiac) left atrium and ventricle.
(More? Cardiovascular System Development | Image - sympathetic plexuses | Neural Crest Development)

posterior neuropore

(caudal neuropore) The opening of the early neural tube at the spinal cord end in the tail region. Initially when the neural plate folds and fuses it leaves two openings at either end cranial and caudal that close later. In humans, the caudal neuropore closes from Carnegie stage 12 (about 26 days) over a day. Failure of posterior neuropore to close can lead to the neural tube defect (NTD) spina bifida.
(More? Neural System Development | Neural System - Abnormalities | Lecture - Early Neural Development)


A term used to describe infants born after 42 weeks clinical gestation. Normal clinical LPM gestation is 40 weeks (which is usually 36-38 weeks post-fertilization development) infants born before 37 weeks are called premature.
(More? Birth)

Pouch of Douglas

(rectouterine pouch or rectovaginal) A female anatomical region describing the portion of the peritoneal cavity lying between the back wall of the uterus and rectum.
(More? Coelomic Cavity Development)


Acronym for Persistent Pulmonary Hypertension of the Newborn.


Acronym for parts per million, used to describe chemical concentrations.
(More? Chemicals)

preantral follicle

(primary follicle) Term used to describe the developmental stage of ovarian follicle development following primordial in describing the sequence primordial follicle, primary follicle (or preantral follicle), secondary follicle (or antral follicle) development within the ovary. In humans, a number of primordial follicles will begin to develop into preantral follicles (primary), some of which will then form secondary follicle (antral follicles), with only a single secondary follicle developing into the preovulatory follicle (Graafian) each menstrual cycle.
Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - preovulatory follicle
(More? Ovary Development | Oocyte Development | Menstrual Cycle | Fertilization | Lecture - Cell Division/Fertilization)

preauricular tag

The skin tags located in front of the external ear opening, are common in neonates and in most cases are normal, though in some cases are indicative of other associated abnormalities.
(More? Hearing - Outer Ear Development | 2009 Lecture - Integumentary Development | Lecture - Hearing Development)

precocial species

Newborn animals that are relatively mature and mobile from birth or hatching, for example guinea pigs and hoofed animals. (altricial is where newborns are immature and not motile).

precocious puberty

(Latin, præcox = maturing early; pubertas = adulthood) In humans, the early (premature) development of the signs of puberty which can occur in both girls (before age 7 or 8) and in boys (before age 9). This condition can have numerous causes, including in girls a rare syndrome of aromatase excess.
(More? Puberty Development | Genital System Development | Ovary Development | Menstrual Cycle)


(toxemia, pregnancy-induced hypertension) Maternal abnormality with high blood pressure and protein in the urine which can occur during pregnancy and may lead to eclampsia, a serious maternal condition leading to seizures or coma.
(More? Placenta Development | Medline Plus - preeclampsia eMJA - Pre-eclampsia: a lifelong disorder)

pregnancy-associated plasma protein-A

(PAPP-A) This placental protein is thought to have several different functions, including preventing recognition of the fetus by the maternal immune system, matrix mineralization and angiogenesis. It is a member of the alpha-macroglobulin plasma protein family, which bind cytokines and specifically cleave a binding protein for insulin-like growth factors (IGF binding protein 4 protease) modulating cytokine activity. Detection of this protein is also used as a first and second trimester diagnostic test for Trisomy 21.
(More? [Pregnancy-associated_plasma_protein-A|Prenatal Diagnosis - PAPP-A]] | Trisomy 21 | Placenta Development)

pregnancy (clinical)

A pregnancy verified by the presence of a gestational sac on ultrasound.
(More? Week 1)

pregnancy-specific glycoproteins

(PSGs) A family of placentally secreted glycoproteins whose concentration in blood increases exponentially until term. These glycoproteins are members of the carcinoembryonic antigen (CEA) family of cell adhesion proteins associated with some tumours as well as the developing embryo/fetus.
(More? Placenta Development)

pre-implantation genetic diagnosis

(PGD) a screening procedure for embryos produced through in vitro fertilization (IVF) for genetic diseases that would generate developmental abnormalities or serious postnatal diseases. Embryonic cells are removed and screened for chromosomal disorders or genetic diseases before the embryo transfer.
(More? Prenatal Diagnosis | In Vitro Fertilization | Week 1)

preimplantation factor

(PIF) Is a 15 amino acid peptide (MVRIKPGSANKPSDD) secreted only by viable embryos and also the placenta. Thought to have a role in early immunity, promoting embryo-decidual adhesion, and regulating adaptive apoptotic processes.
(More? Placenta - Maternal Decidua | Blastocyst)


The term used to describe infants born before 37 weeks and occurs in occurs in 6–10% of all births. Normal clinical gestation is 40 weeks (which is usually 36-38 weeks embryonic development) infants born after 42 weeks clinical gestation are called postmature. Preterm birth is also associated with up to 70% of perinatal mortality.
(More? Birth)

premature luteinization

(PL) An Assisted reproductive Technology (ART) term that refers to a rise in serum progesterone levels on the day of clinical human chorionic gonadotrophin (hCG) administration. Some studies suggest that this rise may subsequently decrease implantation and pregnancy.
(More? In Vitro Fertilization | Menstrual Cycle)

premature rupture of membranes

(PROM) Clinical term for fetal membranes (amniotic and chorionic) rupture any time after 37 weeks gestation and more than an hour before the onset of labor. Preterm premature rupture of membranes (PPROM) is when this rupture occurs before 37 weeks gestation.
(More? Birth | PMID: 19031080)

Pre-Menstrual Syndrome

(PMS) A diverse range of symptoms that appear before menstruation, which apparently occur due to changing levels of sex hormones (progesterone, estrogen) and progesterone's metabolite (alopregnanolone) during the menstrual cycle. The possible PMS symptoms: tiredness, depression, restlessness, insomnia, irritability, emotional, outbursts of anger, headache, dizziness, abdominal pain, breast pain, water retention, diarrhoea, increase in appetite and food craving (sugar, salt). Women can show some or all of these symptoms.
(More? Menstrual Cycle | ABC RN Monday 8 June 1998 - Premenstrual Syndrome)

prenatal screening sensitivity

(detection rate) Prenatal diagnostic test term describing the probability of testing positive on a prenatal screening test if the congenital anomaly is present.
(More? Prenatal Diagnosis)

prenatal screening specificity

Prenatal diagnostic test term describing the probability of testing negative on a prenatal screening test if the congenital anomaly is absent.
(More? Prenatal Diagnosis)

preovulatory follicle

(Graafian follicle or type 8 (>1000 cells)) In the female ovary, the most mature and gonadotropin sensitive developing follicle that will be released at ovulation. In humans, there is only a single follicle every cycle that has been developing over a number of menstrual cycles, other follicles degenerate by the process called atresia.
Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - preovulatory follicle
(More? Ovary Development | Oocyte Development | Menstrual Cycle | Fertilization | Lecture - Cell Division/Fertilization)


Clinical term used to describe age-related hearing loss that occurs in the elderly population (30-35% of adults aged 65 to 75 years). The hearing loss is usually associated with and greater for high-pitched sounds. Recent research suggests that this loss occurs by induction of a mitochondrial Bak-dependent apoptosis in response to oxidative stress.
(More? 2009 Lecture - Hearing | NIDCD | PMID: 19901338)


A birth term, referring to how the fetus is situated in the uterus at the time of birth.
(More? Birth)

presenting part

A birth term, referring to the part of fetus body that is closest to the cervix, vertex presentation (head) is the most common and safest presenting part for birth.
(More? Birth)

presomitic mesoderm

(PSM, unsegmented presomitic mesoderm) The paired strips of paraxial mesoderm lying beside the notochord that at the level of body formation will progressively form somites. This segmentation occurs in a regular rostro-caudal sequence (somitic oscillator) signaled by the Notch and Wnt pathways.
(More? Week 3 | Mesoderm | Lecture - Mesoderm Development | Somitogenesis)

preterm premature rupture of membranes

(PPROM) Clinical term for fetal membranes (amniotic and chorionic) rupture any time before 37 weeks gestation. Premature rupture of membranes (PROM) is when this rupture occurs after 37 weeks gestation and more than an hour before the onset of labor.
(More? Birth - Preterm | Birth | PMID19031080)


A statistical term used mainly in studying disease/disorders and is a measure of the proportion of a population that are disease cases at a point in time. Generally used to measure only relatively stable conditions, not suitable for acute disorders.
(More? Human Abnormal Development)

primary brain vesicle

(primary cephalic vesicle) Term used to describe the early developing cranial neural tube first expanded regions that will later form all the structures of the brain, the caudal remainder of the neural tube will form the spinal cord. These primary vesicles later divide and form into the secondary brain vesicles.
Three primary brain vesicles: prosencephalon (forebrain) - mesencephalon (midbrain) - rhombencephalon (hindbrain)
(More? Neural System Development | Lecture - Early Neural Development)

primary cilia

(primary cilium, non-motile cilia) A cell surface specialization required for signaling, movement of cells and fluid, sensory olfaction and photoreception. Hedgehog (Sonic hedgehog, Shh) signaling and also the downstream hedgehog targets (Gli proteins). These cilia form on embryonic cells by the process of ciliogenesis. The internal microtubule structure differs for primary cilia (microtubule doublets 9+0 axoneme) from motile cilia (9+2 axoneme). Diseases associated with this structure are called ciliopathies. platelet-derived growth factor receptor-alpha (PDGFRalpha) and Wnt signaling
(More? PMID 19602418)

primary cilium

The term used to describe a single primary cilia.

primary follicle

(preantral follicle or type 4 (26-100 cells)) Term used to describe the stage of follicle development following primordial follicle within the ovary characterised by the development of enlarged (cuboidal) layer of granulosa cells around the oocyte. In humans, a number of primordial follicles will begin to develop into primary follicles, some of which will then form secondary (antral follicle), with only a single antral follicle developing into the ovulating follicle (Graafian) each menstrual cycle. Note that some classifications only refer to a primary (preantral) and secondary (antral) stage of follicle development.
Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - tertiary follicle - preovulatory follicle
(More? Ovary Development | Oocyte Development)

primary heart field

(PHF, first heart field, FHF) A mesodermal region that forms the early primary heart field and contribute specific components (left ventricle) of cardiac development (endocardium, myocardium, and smooth muscle). A later forming second heart field forms adjacently and contributes other cardiac structures. Recently suggested that this primary/secondary terminology should be replaced with gene specific expression domains or fields.
(More? Cardiovascular System Development)

primary organizer

The term used originally by Spemann to describe the dorsal lip of the blastopore, meaning that its action established the organization of the entire early embryo.
(More? Embryology History - Hans Spemann | Lecture - Week 3 Development | primitive node | Nobel Laureate- Hans Spemann)

primary spermatocyte

Intermediate cell in spermatozoa development, within the testis seminiferous tubule they lie in the cell layer luminal to the spermatogonia. These large cells enter the prophase of meiosis I, which lasts about 22 days, completion of meiosis I forms the secondary spermatocytes. Cell division from this stage onwards is incomplete with cells remaining connected by cytoplasmic bridges.
Spermatozoa development: primordial germ cell - spermatogonia - primary spermatocyte - secondary spermatocytes - spermatid - spermatozoa
(More? Spermatozoa Development | Testis Development | Fertilization | Lecture - Cell Division/Fertilization)

primary villi

(primary chorionic villi) Term describing the earliest stage of embryonic placenta development. In humans, the conceptus during week 2 this first stage of chorionic villi development consists of only the trophoblastic shell cells (syncitiotrophoblasts and cytotrophoblasts) forming finger-like extensions into maternal decidua. Initially these finger-like projections cover the entire surface of chorionic sac and later become restricted to the placental surface. The villi stages are ongoing as the placenta continues to grow through both the embryonic and fetal development.
Placental villi stages: primary villi - secondary villi - tertiary villi
(More? Placenta Development | Lecture - Placenta)


Clinical term for woman who is pregnant for the first time.

primitive endoderm

(hypoblast) Embryology term used to describe one of the layers formed by the initial blastocyst differentiation of the inner cell mass (embryoblast) into two distinct layers epiblast and hypoblast (primitive endoderm). In human embryo occurs during week 2. In mouse embryo occurs day 3.5 (E3.5) to day 4.5 (E4.5). This primitive endoderm layer is later replaced through epiblast gastrulation forming endoderm.
(More? Week 2 | Blastocyst | Mouse simulation)

primitive reflex

(infantile reflex) Clinical term describing an involuntary neuromuscular response (reflex) that is present at birth and that normally disappears with postnatal development. These reflexes include: Moro reflex, tonic neck reflex, grasp reflex, sucking and rooting reflex, plantar reflex, stepping reflex, Galant reflex, Babinski reflex and parachute reflex. Absence or persistence of this group of reflexes may indicate neural abnormalities or injuries.
(More? Neural Exam Movies)

primitive node

(Hensen's node, primitive knot) The small circular region located at the cranial end of the primitive streak, where gastrulation occurs, and is a controller of this process. The second role is to act as an initial generator of the left-right (L-R) body axis. It is centered around the primitive pit, the site of epiblast extension cranially to form the initial axial process. Region is equivilant to the blastopore in amphibians.
(More? Gastrulation | Week 3 | Carnegie stage 7 | Lecture - Week 3 Development | Neural Development | Hans Spemann)

primitive segment

(somite) Historic terminology for the somite pair seen in early embryonic development.

primitive streak

Visible region on the surface of the early epiblast embryonic bilaminar disc, representing the site of cell migration during gastrulation (cell migration to form endoderm then mesoderm). The primitive streak extends centrally from the primitive node (Hensen's node) to the disc periphery near the connecting stalk. The orientation of the streak establishes the early body axis (rostro-caudal, head-tail). In the human embryo, gastrulation occurs from week 3 through to week 4.
(More? Gastrulation | Week 3 | Lecture - Week 3 Development | Carnegie stage 7)


(Latin primordialis = origin, German, anlage) Term used to describe the earlier or primitive developmental state of a structure, tissue, organ or cell (-blast).

primordial follicle

(small follicle or type 1, 2, 3 (25 cells)) The structure present in the ovary from birth, located in the stroma of the ovary cortex beneath the tunica albuginea. The earliest structure in the sequence of primordial follicle, preantral follicle, antral follicle development within the ovary.
Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - tertiary follicle - preovulatory follicle
(More? Ovary Development | Oocyte Development | Menstrual Cycle | Fertilization | Lecture - Cell Division/Fertilization)

primordial germ cell

(germ cell, gonocyte) Term used to describe the population of cells that will form either the spermatozoa (sperm) progenitor in the embryonic testes or oocyte (egg) progenitor present in the primordial follicle ovary from birth, located in the stroma of the ovary cortex beneath the tunica albuginea. The primordial follicle is the ovary cortical structure containing the oocyte and the surrounding follicular cells.
(More? Genital Development | Oocyte Development | Spermatozoa Development | Lecture - Genital Development | Week 1)


(Latin primordialis = origin, German, anlage) Term used to describe the earlier or primitive developmental state of a structure, tissue, organ or cell (-blast).

proacrosomal granule

During spermiogenesis the spermatozoa initially modify the Golgi apparatus to form small vesicles filled with carbohydrate and enzymes that coalesce to form the initial acrosome structure.
Golgi apparatus - proacrosomal granule - acrosomal compaction - acrosome
(More? Spermatozoa Development | Fertilization)

processus vaginalis

A transient communicating channel in testes development between tunica vaginalis and peritoneal cavity. In females this is the "canal of Nuck".
(More? Testis Development | Lecture - Genital Development | testis descent image)


In most female mammals, the first stage in the estrous cycle immediately before estrus characterized by development of both the endometrium and ovarian follicles.
Estrous cycle stages: proestrus - estrus - metestrus - diestrus
(More? Estrous Cycle | Mouse Estrous Cycle)

prophase I

First phase of meiosis I, which together with meiosis II results in the reductive cell division only occurring gametes. Prophase can be further divided into a number of stages: leptotene zygotene, pachytene, diplotene, diakinesis.
(More? Cell Division - Meiosis)


A steroidal hormone, of the progestogens class, produced by granulosa cells in the ovary that has many endocrine roles in the female. Other biological sources include: adrenal glands, gonads (corpus luteum), brain, and placenta. Male progesterone has a suggested role in neural development. Female functions include regulation of the menstrual cycle, uterine changes, maintaining pregnancy and effects on systems throughout the body. Recently this hormone has also been identified as a chemoattractant for spermatozoa during fertilization. Progesterone is also used clinically as a part of hormone replacement therapy (HRT) in women. Hormone is bound by the progesterone receptor, a nuclear hormone receptor transcription factor.
(More? Menstrual Cycle | Spermatozoa Motility)

progesterone receptor

A nuclear hormone receptor for progesterone that with steroid hormone bound (ligand-activated) acts as a gene transcription factor. This receptor family has 4 molecular domains: 1. C-terminal ligand-binding domain (binds the steroid), 2. a hinge region, 3. a DNA binding domain (binds to response element on the DNA), and 4. an N-terminal domain (for transcriptional activation). Human gene is located on chromosome 11 at 11q21-q23 and there are two receptor isoforms (PRA and PRB) that differ by an additional 165 amino acids on the N-terminal of PRB.
(More? Menstrual Cycle | OMIM - progesterone receptor)


These compounds are synthetically produced reproductive progestogens used clinically and experimentally.
(More? Menstrual Cycle)


(PRL) anterior pituitary hormone which stimulates breast development and milk production in pregnancy. Also has a role in regulating follicle stimulating hormone (FSH) effect on the ovary. Protein hormone is similar in structure to both growth hormone (anterior pituitary) and chorionic somatomammotropin (placenta). Anterior pituitary release of prolactin is in turn regulated by the hypothalamus prolactin-releasing hormone (PRLH, prolactin-releasing peptide). Recently been shown to to mimic in pregnancy effects of increased maternal myelination processes (oligodendrocyte precursor proliferation).
(More? Endocrine - Pituitary Development | Mammary Gland Development | OMIM - PRL)

prolactin-releasing hormone

(PRLH, prolactin-releasing peptide, PRRP) an 87 amino acid peptide hypothalamus hormone which regulates anterior pituitary release of prolactin.
(More? Endocrine - Hypothalamus Development | OMIM - PRLH)


(Greek, pro = before) Cell division term referring to the second mitotic stage, when the nuclear envelope breaks down into vesicles. Microtubules then extend from the centrosomes at the spindle poles (ends) and reach the chromosomes. This is followed by metaphase.
Mitosis Phases: prophase - prometaphase - metaphase - anaphase - telophase
(More? Cell Division - Mitosis | Cell Division - Meiosis | Week 1)


(Greek, pro = before) The first temporary stage of kidney development (pro-, meso-, meta-). This forms the kidney of primitive fish and lower vertebrates. Kidney development occurs within the intermediate mesoderm interacting with endoderm. In humans, this very rudimentary kidney forms very early at the level of the neck. It is rapidly replaced by the mesonephros, intermediate stage kidney, differentiating in mesoderm beneath.
Kidney Stages: pronephros - mesonephros - metanephros
(More? Renal System Development | Lecture - Renal Development)

pronuclear fusion

(Greek, pro = before) The process of the fusion of the two haploid nuclear structures (pronuclei) contributed from the spermatazoa and oocyte to form the first diploid nucleus cell. Can also be called fusion of pronuclei.
(More? 2009 Lecture - Cell Division and Fertilization | Week 1)


(Greek, pro = before) The two haploid nuclei or nuclear structures containing the genetic material from the spermatozoa and the oocyte. These two haploid nuclei will fuse together to form the first diploid nucleus cell, the zygote. Therefore the nuclear structures that exist "before the nucleus", the plural term is pronuclei.
(More? Carnegie stage 1 | Week 1)


The thin skin membrane extending from the shoulder to the wrist anterior to the upper arm and forearm forming the leading edge of the bat wing.
(More? Bat Development)


(Greek, pro = before) Cell division term referring to the first mitotic stage, when the diffusely stained chromatin resolves into discrete chromosomes, each consisting of two chromatids joined together at the centromere.
Mitosis Phases: prophase - prometaphase - metaphase - anaphase - telophase
(More? Cell Division - Mitosis | Cell Division - Meiosis | Week 1)


(Greek, pro = before) The neurological sense of body position that is perceived both at the conscious and unconscious levels. There are a variety of peripheral specialized sensory structures that together provide the information on body position in space.
(More? Neural Development)

proprotein convertase 5/6

(PC5/6) A protein identified as important for human and mouse embryo implantation, regulating both uterine epithelial receptivity and stromal cell decidualization.
(More? Implantation)


In late spermatogenesis, these small nuclear proteins replace the normal histones as the packing protein for DNA condensation and stabilization.
(More? Spermatozoa Development)

protease nexin-1

(PN-1) A serine protease inhibitor that has been shown to regulate in the neural system NMDA receptor (NMDAR) activity and sonic hedgehog signaling. The sonic hedgehog signaling affects the neural progenitor proliferation in the hippocampus.
(More? Molecular Development | PMID20529321)


A cell biology term describing a large multiprotein complex found in all cells that functions in degradation of ubiquitylated proteins.
(More? UNSW Cell Biology)


The biological molecule produced by translation of messenger RNA (mRNA) which in turn is the product of genes (DNA). Proteins are therefore often described as the products of gene expression and are a key component of all cell functions.
DNA -> RNA -> Protein

protein kinase

A protein enzyme that add phosphate groups to substrates, mainly other proteins, and has the opposite enzymatic activity to a protein phosphatase. Protein phosphorylation is the most common form of protein post-translational modification and can alter the target protein function or activity. Protein kinases can be part of membrane receptors and also cytoplasmic as part of signaling cascade and other pathways.

protein phosphatase

A protein enzyme that remove phosphate groups from substrates, mainly other proteins, and has the opposite enzymatic activity to a protein kinase. Phosphatases are generally cytoplasmic and part of signaling and other pathways.


The abnormal presence of protein in the urine and an indicator of diesease including diabetic kidney disease (DKD, diabetic nephropathy).
(More? Renal System - Abnormalities)

protocadherin 15

(Pcdh15) A protein required for initial formation of stereocilia bundles and changes in the actin meshwork within hair cells. The Ames waltzer (av) mouse mutant has both auditory and vestibular abnormalities from a mutation in this gene.


(forebrain) The most anterior neural tube primary brain vesicle (there are 3 primary brain vesicles) that will form the two secondary brain vesicles, telencephalon and diaencephalon. These generate in the adult brain the cerebral hemispheres (neocortex, basal nuclei, palaeocortex, archicortex) and thalmus, hypothalmus and other nuclei respectively. The prosencephalon lumen (cavity of the neural tube) will form the lateral ventricle and third ventricle.
Primary brain vesicles: prosencephalon (forebrain) - mesencephalon (midbrain) - rhombencephalon (hindbrain)
(More? Neural - Prosencephalon Development | Lecture - Early Neural Development | Neural Development)


A family of hormone-like substances derived from arachidonic acid that have regulatory roles in a range of physiological functions: contraction and relaxation of smooth muscle; the dilation and constriction of blood vessels; control of blood pressure; and modulation of inflammation. The name is a historical misnomer, originally isolated from seminal fluid in 1935, prostaglandin was thought to originate from the prostate gland though it is produced by the seminal vesicles. In the early 1970s identified as a mediator of uterine activity associated with labor and birth.

prostaglandin E2

(PGE2, dinoprostone) This prostaglandin is important during birth for softening of the cervix (cervical ripening) and stimulating uterine smooth muscle contraction.
(More? Birth)

prostaglandin F2 alpha

(PGF2 alpha, dinoprost) This prostaglandin is important during birth for induce labor and stimulating uterine smooth muscle contraction. The labor inducing activity can also be used clinically as an abortifacient.
(More? Birth)

postnatal period

Term refers to the period after birth and covers the perinatal and neonatal periods through to about 6 weeks after birth.

postneonatal period

Term refers to the period extending from the end of the first month to a year after birth.


(Latin, prurire = to itch) itching skin sensation which can have a number of different causes (diabetes, iron deficiency, hyperthyroidism, Hodgkin's Disease, hair loss, allergic drug reaction).
(More? Lecture - Integumentary Development | Integumentary Development)


(PSACH) Clinical autosomal dominant skeletal dysplasia disorder characterized by: short-limbed dwarfism, epi-metaphyseal dysplasia, joint laxity and early onset osteoarthritis. Histologically, there is a generalized epi-metaphyseal dysplasia. (OA) OMIM 177170

pseudoglandular stage

Term used to describe the second histological/developmental stage of lung development, after early embryonic. In humans this stage occurs during the early fetal period after about 20 generations of branching.
Lung stages: respiratory embryonic stage - pseudoglandular stage - canalicular stage - terminal sac stage - alveolar stage
(More? Respiratory System Development | Lecture - Respiratory Development)


(Disorders of Sex Development, DSD) A historic terminology no longer applied and replaced with the term Disorders of Sex Development (DSD). Humans having both male and female reproductive organs, occurs in both male and female forms.
  • 46,XY DSD karyotype who have a predominantly female phenotype (blind-ending vagina, absence of breast development, primary amenorrhea, presence of testicular structures) that is with gonads of one sex, external genitalia of opposite.
  • 46,XX DSD karyotype with gonads as ovaries, external genitalia ambiguous and hyperplastic adrenals secreting androgens.
  • Ovotesticular DSD
(More? Genital System - Abnormalities)

pubertal arrest

(Latin, pubertas = adulthood) An abnormal timing of puberty, where there is no progress in puberty over 2 year period. See also delayed puberty.
(More? Puberty Development | Genital System Development | Ovary Development | Menstrual Cycle)


(Latin, pubertas = adulthood) The process involving maturation of the reproductive system and development of secondary sex characteristics. A complex process, initiated by an unknown mechanism, but involving the brain driving the hormonal axis. In humans, female puberty initiates at an earlier age than male puberty.
(More? Puberty Development | Endocrine - Gonad Development | Lecture - Endocrine Development | Lecture - Genital Development)


Term pertaining to childbirth or the period immediately following birth.
(More? Birth)


A maternal term referring to the six weeks following parturition (birth) when maternal reproductive organs and physiology return to the pre-pregnant state.
(More? Birth)

purulent meningitus

Bacterial infection by E. coli or streptocci B. can cause vascular thrombosis involving the choroid plexus, which in turn can alter cerebrospinal fluid (CSF) flow leading to hydrocephalus.


The dilatation of the fetal renal pelvis (anteroposterior pelvic diameter >/=4 mm) visible with ultrasound.
(More? Renal Development | Ultrasound)


(Greek, pyknon = dense) term used to describe a DNA non-random pattern of repeated sequence elements identified in the human genome following computer analysis (genic and nongenic regions, patterns of 16+ bases in length, occurring 40+ times in nongenic). These sequences are: found frequently in the 3' untranslated regions (3'UTR) of genes, can be repeated with a regular spacing of 18 to 22 nucleotides between copies, and have currently an unknown function.


An acute or chronic infection of the uterus with accumulation of pus in the uterine lumen. This is a relatively common condition in other species such as dogs, cats, cows, and mares. In rats, it can occur due to a genital tract infection with Mycoplasma pulmonis.


A type II membrane glycoprotein component found in fenestrae and in caveolae.
(More? PMID 10557298)

Glossary Comments

Use this page to access brief definitions of specific embryology terms. Additional information can be accessed from links listed at the end of each definition. Glossary from the UNSW Embryology program compiled and written by Dr Mark Hill. Reference material used in preparing this glossary list includes: texts listed on page 1 "Reading" of each notes section, Department of Anatomy Publications, WWW resources from NCBI, NIH, OMIM, NHMRC (Australia), AMA (USA), Office of Rare Diseases (USA), PubMed Medline Dictionaries, MSDS, Merck Manual home edn. and WHO ART terminology (2009).

These notes are for Educational Purposes Only Please email Dr Mark Hill if you wish to make a comment about this current project.

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Cite this page: Hill, M.A. 2017 Embryology P. Retrieved October 23, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/P

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