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Glossary Links

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link



A genetics term used to describe a technique for staining chromosomes during metaphase. The resulting banding pattern is the opposite of G-banding and there are several other chromosome staining techniques.
(More? Molecular Development - Genetics)

radial hemimelia

Clinical term for a congenital abnormality characterised by the partial or complete absence of the radius.
(More? Limb Abnormalities | PMID 18950501)


Anatomical line of fusion of the urogenital folds lying along the urethra and scrotum in male external genitalia.
(More? Genital System Development)

Rathke's pouch

An ectodermal fold in roof of pharynx forming anterior pituitary (adenohypophysis) and pars intermedia. Named after German embryologist and anatomist Martin Heinrich Rathke (1793 -1860).

(More? Endocrine - Pituitary Development | Endocrine Development | Martin Heinrich Rathke)

Rauber's layer

A thinned-out trophoblast membrane over the embryonic disk in developing carnivores and ungulates. In cattle, prevention of the loss of the polar trophoblast layer leads to ectopic domains of BRACHYURY, a gastrulation marker. Named after August A. Rauber (1841-1917) a German anatomist.
(More? trophoblast | PNAS)


General term generally for the cellular proteins which specifically bind a ligand or a number of ligands in signal transduction. Associated in many forms of signaling including: developmental, neural, neuromuscular and endocrine signaling. Receptors can be found embedded in cell membranes, cytoplasmic, nuclear and on organelles.

receptor protein tyrosine phosphatase

(RPTP) A family of enzyme phosphatases that control aspects of nervous system development. Phosphatases remove phosphate groups from substrates, mainly proteins, and have the opposite enzymatic activity to kinases. Drosophila have 6 different RPTPs including leukocyte antigen-related (LAR) involved in photoreceptor axon targeting independent of its phosphatase activity.
(More? PMID 19889974)

receptor tyrosine kinase-like orphan receptor

(Ror2, Neurotrophic tyrosine kinase, receptor-related 1, NTRKR1) A cell surface receptor protein suggested to interact with wnt5a in developmental signal transduction. The receptor activates multiple downstream intracellular mediators.
(More? OMIM 602336)
rectouterine pouch
rectouterine pouch

rectouterine pouch

(Pouch of Douglas or rectovaginal) Anatomical description of the female peritoneal cavity lying between the back wall of the uterus and rectum.
(More? Coelomic Cavity Development | Uterus Development

recurrent pregnancy loss

(RPL) Clinical term for 3 or more consecutive miscarriages and is suggested as due to a range of maternal and embryonic causes.

reduced fetal movement

A diagnostic term for referring to an analysis of fetal movements during the third trimester. There are a wide range of different methodologies and criteria associated with this technique.
(More? Third Trimester | Ultrasound)

reductive division

Cell division term describing meiosis where diploid DNA content becomes haploid (halved).
(More? Meiosis)


(Reln, RL) A protein required for neuronal migration and cellular layer formation during brain development. Mutation in the mouse model lead to impaired motor coordination, tremors, and ataxia. Reelin is an extracellular protein involved in signaling through the Dab1 adapter protein, and Lis1 regulate neuronal migration and cellular layer formation in the brain. The genomic structures of the mouse and human RELN genes appear to be highly conserved and the loss of Reelin and reduction in Lis1 activity in both mice and humans results in the disorganization of cortical structures.
(More? Neural System Development | OMIM - REELIN)

regulatory sequence

(regulatory region, regulatory area) Genetic term describing a segment of DNA where regulatory proteins such as transcription factors can bind preferentially. The regulated gene can either increase or decrease expression following binding of these factors.
(More? Molecular Development)
Reichert's cartilage
Reichert's cartilage

Reichert's cartilage

(second pharyngeal arch cartilage) Neural crest within the second pharyngeal arch forming this cartilage band. The dorsal ends form the middle ear ossicle (stapes) and temporal bone styloid process, the ventral part ossifies to form hyoid bone components of the lesser cornu and superior body. A study (PMID 16441562) suggest that this is not a continuous band (like Meckel’s) but a series of segments and mesenchymal tissue. Named after Karl Bogislaus Reichert (1811 – 1883) a German anatomist.
(More? Middle Ear | Head Development)

Reichert's membrane

In early rodent development, a thick multilayered basement membrane between the parietal endoderm cells and the trophoblast cells. This membrane has contentious origins, and has been suggested to begin with the formation of a basement membrane of the mural trophoblast cells, subsequent growth is from primitive endoderm cell depositions.
(More? Mouse Development | PMID 8651512)

relative risk

(risk ratio or odds ratio) Term used in describing the effect of environmental teratogens on development. The ratio of the rate of the condition among the exposed and the non-exposed population. (e.g. smokers risk of having a low birth weight baby compared to non-smokers) A high relative risk may indicate a low absolute risk if the condition is rare. The other risk descriptor term used to describe the likelyhood of an abnormality is absolute risk.
(More? Environmental Abnormal Development)


The reproductive hormone involved in uterine relaxation (some species), reproductive tissue growth, connective tissue remodeling (collagen), cardiovascular, renal system, and brain. Relaxin receptor is G-protein-coupled receptor (relaxin family peptide receptors 1 - 4, RXFP1-4).
(More? Genital System Development)
Early fetal kidney
Renal (=kidney)


(Latin, renes = "kidney") Term used in relation to the kidney and associated structures (renal pelvis, renal artery)
(More? Renal System Development | Lecture - Renal Development)


Term used in relation to the heart, repolarization is a term used to describe the relaxation after each cardiac contraction which is controlled by a gradient of ion channels.
(More? Cardiovascular System Development)


A commercial online database that contains summaries on the effects of medications, chemicals, infections, and physical agents on pregnancy, reproduction, and development.
(More? REPROTOX | Abnormal Development - Environmental)


Term used in relation to breathing (in and out) or associated with the lungs. Anatomically used to describe the lungs, air pathways and associated muscles. In cell biology used in relation to mitochondrial use of oxygen to produce energy and carbon dioxide waste.
(More? Respiratory System Development)


A national, nonprofit consumer organization offering education, advocacy, and support to those experiencing infertility. Services include a national HelpLine, quarterly newsletter, extensive literature list, member-to-member contact systems, and local support groups through a network of over 50 chapters nationwide.

resorptive bay

Osteoclast resorptive bay
Osteoclast resorptive bay
(Howship's lacuna) The histological name for the shallow bay or cavity lying directly under an osteoclast located at the site of bone remodeling. This microscopic extracellular matrix space represents the site of bone matrix resorption by the osteoclast.
(More? Histology Image - Osteoclast | Bone Histology | Bone Development Practical | Lecture - Musculoskeletal Development)

respiratory embryonic stage

respiratory embryonic stage
respiratory embryonic stage
(lung bud stage) The earliest stage of lung development when the two lungs are primitive "buds" growing into the future pleural cavity.
Lung Stages: respiratory embryonic stage - pseudoglandular stage - canalicular stage - terminal sac stage - alveolar stage
(More? Respiratory System Development | Lecture - Respiratory Development)

respiratory tree

Anatomical term to describe the components of the respiratory system (lungs) as they branch again and again ending in the functional units, the alveolar sacs (alveolus).
(More? Respiratory System Development)

retained products of conception

Where part of the placenta or fetal membranes remains within the uterus, often associated with early fetal loss in the first trimester (previously called incomplete abortion).

rete ovarii

(Latin, rete = "net", refers to a network) A group of epithelial tubules located at the hilum of the ovary possibly mesonephric origin.
(More? Ovary Development)

rete ridge

(Latin, rete = "net", refers to a network) Integumentary histology term describing the extensions of the epidermis into the dermis. These epidermal surface thickenings extend downward between underlying connective tissue dermal papillae. This is also the site of initial eccrine gland differentiation.
(More? Integumentary System Development)

rete testis

(Latin, rete = "net", refers to a network) The duct (epithelial tubules) conduction system for spermatazoa embedded within the mediastinum (connective tissue) located in the center of the testis (male gonad) derived from the mesonephric duct, and allow spermatazoa to travel from the seminiferous tubules to the vasa efferentia.
Spermatozoa Duct Pathway: seminiferous tubule - straight tubule - rete testis - ductuli efferentes - ductus epididymidis - ductus deferens
(More? Lecture - Genital Development)


The stratified sensory structure of the eye, formed from the neural ectoderm that extends from the forebrain (diencephalon) to form initially the folded optic cup. Vertebrates have ten identifiable layers formed from nerve fibers, neurons, membranes, photoreceptors and pigmented cells. Light must pass through nearly all these layers to the photoreceptors.
(More? Retina Development | Sensory - Vision Development)

retinal pigment epithelium

(RPE) An epethial pigmented cell layer lying outside the sensory retina, formed from the outer layer of the folded optic cup. The RPE is firmly attached to the underlying choroid and overlying retinal visual cells, for which it has a nutritional role.
(More? Retina Development | Sensory - Vision Development)

retinal waves

A form of coordinated spontaneous activity that occurs in the developing retina. These waves of electrical activity (action potentials) along with EphA/ephrin-A signaling are thought to have a role in establishing the initial retinotopic map by correlating/coordinating the activity of neighbouring retinal ganglion cells.
(More? Retina Development | Sensory - Vision Development)

retinopathy of prematurity

(ROP) A treatable vascular proliferative disorder that affects the incompletely vascularized retina in premature neonates, birth weight 1250 grams or less and born before 31 weeks gestation GA are at highest risk. Classified as type 2 progressing to type 1, this is a primary cause of childhood blindness. Due to retinal immaturity, neovascularization occurs leading to retinal traction and retinal detachment, eventually affecting vision.
(More? Vision Abnormalities | Birth - Preterm | Sensory - Vision Development | NIH - ROP | American Association for Pediatric Ophthalmology)

retrograde menstruation

Term describing the passage of menstrual blood into the uterine tubes and peritoneal cavity during menses.
(More? Menstrual Cycle)

Rett syndrome

(RTT) A severe neurodevelopment disorder, with intellectual disability and abnormalities of movement, mainly caused by mutations in the X-linked (Xq28) Methyl-CpG-binding protein 2 (MECP2) gene and therefore almost exclusively in females. The congenital variant of Rett syndrome is caused by heterozygous mutation in the FOXG1 gene on chromosome 14q13.
(More? Neural System - Abnormalities | Neural System Development | OMIM - Congenital Rett syndrome | OMIM - Rett syndrome)

Rhesus D

(RhD, Rh factor) See Rh factor below. The protein on surface of red blood cells in some blood types (Rh+) and absent in others (Rh-). Can cause erythroblastosis fetalis in second pregnancy if fetal/maternal blood of opposite groups mix on first pregnancy.

rheumatic fever

Term for an acute disease that affects mainly children and young adults. It is a reaction to a throat infection by a particular bacteria and can damage the: heart (rheumatic heart disease, damaging muscle, valves and lining), joints and the brain.
(More? Cardiovascular System Development)

rheumatoid arthritis

A chronic inflammatory autoimmune disease in which the immune system attacks the tissues lining the joints. The inflamed joints often cause pain, heat and swelling and can lead to functional limitations and severe disability.

Rh factor

(Rh) The protein on surface of red blood cells in some blood types (Rh+, Rh positive) and absent in others (Rh-, Rh negative). Can cause fetal erythroblastosis (erythroblastosis fetalis or Haemolytic Disease of the Newborn) in second pregnancy if fetal/maternal blood of opposite groups mix on first pregnancy. Clinical guidelines in some countries recommend anti-D immunoglobulin therapy for all pregnant women who are RhD negative, regardless of the rhesus status of the fetus. Non-Invasive Prenatal Testing (NIPT) may in future allow testing of the fetal rhesus status.
(More? Blood Development | Non-Invasive Prenatal Testing)


A small GTPase of the Rho subfamily (Rho, Rac and Cdc42) acts as regulatory switch for actin cytoskeleton. In development, activated in epithelial cells for invagination.
(More? Molecular Development)


(Greek rhombos = rhomboid, enkephalos = brain, hindbrain) The hindbrain, the most caudal neural tube primary brain vesicle (there are 3 primary brain vesicles) that will form the two secondary brain vesicles, the metencephalon and the myelencephalon. These will in turn generate in the adult brain the cerebellum, pons and medulla. The rhombencephalon lumen (cavity of the neural tube) will form the fourth ventricle.
Primary brain vesicles: prosencephalon (forebrain) - mesencephalon (midbrain) - rhombencephalon (hindbrain)
(More? Lecture - Ectoderm Development)


Hindbrain rostrocaudal segmentation established by segmental expression of Hox homeodomain transcription factors. Histologically rhombomeres are visible as undulating folds (scalloping) of the neural tube in the hindbrain region and have associated cranial nerves.
(More? Lecture - Neural Development | Lecture - Ectoderm Development)


A type of molecular regulation in bacteria where an RNA element binds metabolites and then regulates gene expression. This type of regulation is usually cis, where a 5' untranslated region (5'UTR) riboswitch modulates transcription or translation of the downstream coding sequence.

rich club

A new neural pathway term that refers to brain regions that form a high-cost, high-capacity backbone thought to enable efficient network communication. This has been shown to differ in pre-term infants.
(More? Neural System Development | Preterm | PMID 24799693)


A highly potent toxin produced by castor beans, it contains a ribotoxic A chain (RTA) and a cell-binding B chain.

RNA retrovirus

A virus with genetic material stored in the form of RNA that can replicate in a host cell and then by reverse transcriptase produce DNA that is then incorporated into the host's genome.
(More? Abnormal Development - Viral Infection)

RNA silencing

(RNA interference, RNAi, microRNA, miRNA) Molecular development term describing the complex of mechanisms that regulate gene expression through small RNA molecules, in mammals this is mainly the microRNA (miRNA) pathway. These short RNA molecules, produced by RNAse III family enzyme Dicer, guide sequence-specific silencing of gene expression to suppress mRNA translation into protein. This mechanism has also been identified in early development during week 1 in zygote to blastocyst formation.

(More? Molecular Development)

Robinow syndrome

A skeletal dysplasia characterized by short stature, limb shortening, genital hypoplasia, and craniofacial abnormalities. Has both autosomal dominant and autosomal recessive (original family described by Meinhard Robinow) inheritance patterns. Recently, two different missense mutations in Wnt5a have been identified with the autosomal dominant Robinow syndrome, resulting in decreased Wnt5a activity.
(More? OMIM 268310)

Robson classification

(10 group classification) A clinical term for a caesarean delivery classification system first described by Robson in 2001.
(More? Caesarean Classification | caesarean | PMID 11359322)

roof plate

The term used to describe the thin dorsal region of the early neural tube forming the central nervous system (brain and spinal cord). The roof plate also lies adjacent to the dorsal ectoderm and initial neural crest prior to migration. The opposite side of the early neural tube forms the floor plate and the lateral parts are divided into two thicker regions, the alar plate and basal plate.
(More? Lecture - Ectoderm Development | Neural System Development)


An acronym for receptor tyrosine kinase-like orphan receptor.
(More? Molecular Development

rostral neuropore

(cranial, cephalic or anterior) See cranial neuropore.
(More? Neural System Development)


Anatomical axis term used to describe a head (rostral) to tail (caudal) direction or sequence of developmental events. Some embryonic structures form initially in a rostral (head-ward) region and later similar structures form more caudal (tail-ward). Examples include somite formation (somitogenesis), limb and pharyngeal arch development.


A type of virus that is a common cause of diarrhoea and vomiting in infants and young children. The live attenuated rotavirus vaccine is contraindicated in pregnancy, but can be safely administered to household contacts of pregnant women. There is only a very small risk of transmission of the rotavirus vaccine virus to a susceptible pregnant woman and there is no evidence of risk to the fetus if pregnant women are in contact with recently vaccinated individuals. (data based on: The Australian Immunisation Handbook 9th Edition 2008 2.3.2 Vaccination of women planning pregnancy, pregnant or breastfeeding women, and preterm infants - updated July 2009 )
(More? Viral Infection | 2.3.2 Vaccination of women planning pregnancy, pregnant or breastfeeding women, and preterm infants - updated July 2009 | The Australian Immunisation Handbook 9th Edition 2008)

RU 486

(mifepristone) Clinical potent hormone progesterone and glucocorticoid receptor antagonist used as a birth control drug. Progesterone binding to the progesterone receptor generates a receptor conformational change allowing it to bind to DNA and act as a transcription factor for genes. RU486 binds with 10-fold higher affinity the same receptor C-terminal region of the hormone-binding domain and then does not act as a transcription factor.
(More? Mifepristone)


Infant rubella virus
Infant rubella virus
(Latin, rubella = little red) Virus also known as "german measles" (due to early citation in German medical literature). This virus can crosses the placenta from infected mothers and leads to major developmental defects in many different developing systems, including vision and hearing.
(More? Abnormal Development - Rubella Virus)

rugae palatinae

Mouse palatal rugae
Mouse palatal rugae
(palatal rugae) The transverse ridges forming on the secondary palate which are also sequentially added as the palate grows.
(More? Palate Development | Cleft Palate | Cleft Lip and Palate | Head Development)


(Acute Myeloid Leukemia 1, AML1) Transcription factor of the Runt-related protein family (3 members in the mammalian family). Runx1 can act as a context-dependent transcriptional activator or repressor. In neural development, expressed in post-mitotic neurons and plays important roles in both phenotype specification and axonal targeting. AML1 is one of the most frequent targets of chromosome translocations associated with leukemia.
(More? Neural System Development | Molecular Development | OMIM - Runx1)

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.

Glossary Links

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Cite this page: Hill, M.A. (2024, May 25) Embryology R. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/R

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G