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hair follicle cartoon
Hair follicle

hair follicle

The specialised integumentary (skin) hair forming structure with both epithelial and mesenchymal origins.
(More? Hair Development | Integumentary System Development)

Hamburger Hamilton Stages

Chicken stages of development named after the 2 authors of a paper that divides the 21 days of chicken embryo development into 46 defined stages. These were published in: Series of Embryonic Chicken Growth. J. Morphology, 88 49 - 92 (1951).
(More? chicken | Hamburger Hamilton Stages)


Term used to describe species where one sex is haploid and the other sex is diploid in chromosome number. Sexual reproduction can therefore regulate offspring sex. For example in haplodiploid arthropods, in which the sex determination mechanism (males are haploid and females are diploid) allows the female to control the sex of their offspring.
(More? fertilization)


(Greek, haploos = single) Having a single set of chromosomes as in mature germ/sex cells (oocyte, spermatozoa) following reductive cell division by meiosis. Normally cells are diploid, containing 2 sets of chromosomes. Ploidy refers to the number of sets of chromosomes in the nucleus of a cell.
(More? meiosis | spermatozoa | oocyte)
hard palate
hard palate

hard palate

The bony anterior portion of the palate formed by maxillary and palatine bones. The posterior muscular posterior portion is called the soft palate.
Palate Links: palate | cleft lip and palate | cleft palate | head | Category:Palate


Term used to describe the colon recesses or sacculations caused by contractions of the teniae coli and circular muscle.
(More? Gastrointestinal Tract Development)

haemal arch

Term referring to the axial skeleton bony arch region within tail vertebra that contain blood vessels. General vertebrae anatomy is composed of the neural arch (containing the spinal cord), the haemal arch and the vertebral body (the centrum).
(More? Axial Skeleton | Bone Development)


The percentage by volume of red blood cells in whole blood.
(More? blood)

haematopoietic stem cell

(hematopoietic stem cell, HSC) The stem cell population that generates (haemopoiesis) all the cells found in blood (red blood cells and white blood cells). In the embryo these stem cells are mesoderm in origin, located initially in the yolk sac then the aorta-gonad-mesonephros region, embryonic liver, and fetal bone marrow. In the adult these cells reside in the bone marrow.
(More? Image - Hematopoietic and stromal cell differentiation | Blood Development | Liver Development)
Hematopoietic and stromal cell differentiation


(hematopoiesis) The term used to describe the process of blood cell formation/differentiation from blood stem cells (haematopoietic stem cells). In the embryo, this is mainly red blood cell formation which begins in the yolk sac mesoderm, then the liver and other organs, finally residing in the adult in the bone marrow.
(More? Image - Hematopoietic and stromal cell differentiation | Blood Development | Liver Development)

Haemolytic Disease of the Newborn

See fetal erythroblastosis.


A gene encoding a basic helix-loop-helix (bHLH) transcription factor that is expressed in fetal and adult heart. In the mouse, Hand1 and Hand2 are expressed in the heart and some neural crest derivatives, and Hand1 is also expressed in extraembryonic membranes.
(More? OMIM: HAND1)


These abnormalities are benign focal malformations that resemble a neoplasm in the tissue of its origin and are often seen in association with tuberous sclerosis.


An acronym for Haplotype Map, an international project to catalogue the human genome single nucleotide polymorphisms (SNPs), the variation in a single DNA nucleotide that occurs at a specific position in the genome.
(More? Genetic | HapMap)

harderian gland

A gland located in vertebrate (reptiles, amphibians, birds and mammals) eyes that varies in secretion and functions between species. The gland also has associated melanocytes. The gland is named after Johann Jacob Harder (1656 - 1711) a Swiss anatomist.

Harrington rod

(Harrington implant) The first modern (1953) spinal instrumentation system that produced both correction and immobilization for spinal abnormalities (scoliosis). This historic system has now been replaced by more modern systems. Dr Paul Harrington (1911 – 1980) was an American orthopaedic surgeon who designed the system that remained in service until the 1990's. See also the Chêneau brace.
(More? Axial Skeleton Development | Musculoskeletal System - Abnormalities)

Hassall's bodies

(Hassall's corpuscles) Thymus histological structures that appear in fetal development and increase in number until puberty, then decreases. Named after Arthur Hill Hassall (1817-1894) a British physician and chemist.
(More? Endocrine - Thymus Development)

Haversian canal

Bone histology historic term used to describe the small central canal in the Haversian system bone microstructure that contains blood vessels for osteocyte nutrition and nerves for sensation. These canals are linked by V#Volkmann's canals to the periosteum. Canal was named after Clopton Havers (1650-1702) an English physician and anatomist.
(More? Bone Histology | Bone Development)

Haversian system

(osteon) The term used to describe the histological and micro-anatomical unit structure (principal structure) of compact bone. Consists of a central cavity surrounded by lamellar bone matrix within which osteocytes reside. The central cavity or canal (Haversian canal) contains blood vessels for osteocyte nutrition and nerves for sensation.
(More? Bone Histology | Bone Development | Lecture - Musculoskeletal Development)


Acronym for heparin-binding epidermal growth factor (EGF)-like growth factor.


An acronym for the ultrasound measurement of Head Circumference.


(human Chorionic Gonadotrophin) An acronym for the placental hormone human Chorionic Gonadotrophin.
(More? Human Chorionic Gonadotropin)

hCG ratio

Clinical term for the measure of the increase or decrease in human Chorionic Gonadotrophin (hCG) levels, calculated by dividing the current measured hCG concentration by the previous measurement.
(More? Human Chorionic Gonadotropin)

head circumference

An ultrasound measurement of Head Circumference (HC) is used to determine fetal age and normal development (small/large/abnormal) parameters. Measured as an ellipse in a horizontal section at the level of the thalamus and the cavum septi pellucidi. It is one of the four typical ultrasound assessments of fetal size and age: Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).
(More? Fetal head growth graph | Postnatal growth charts | Movie - Measuring newborn head circumference | Ultrasound | Head | Fetal Development | Birth)


(cardiac) An early developing organ which fuctions as a pump for blood in the embryo and adult. In the human, the heart development appears in week 4 within the splanchnic mesoderm as a simple tube. This tube then undergoes a series of growth, complex folding and reorganization to form the 4 chambered heart. There are many developmental abnormalities associated with heart development.
(More? Cardiovascular System Development | Lecture - Heart Development | Cardiac Embryology Online Tutorial)

heart field

The term used to describe the splanchnic mesoderm cardiogenic region in the trilaminar embryo that generates most of the heart. In humans, there are two fields the primary and secondary heart fields.
(More? Cardiovascular System Development | Lecture - Heart Development | Cardiac Embryology Online Tutorial)

heart prominence

Refers to the readily visible surface enlargement of the embryo ventral body wall when the heart begins to develop (week 4).

(More? Cardiovascular System Development | Lecture - Heart Development | Cardiac Embryology Online Tutorial)

heart valves

The heart has a series of valves which regulate the directional flow of blood. The human heart has valves separating the atria from ventricles (atrioventricular, AV) and the ventricles from the outflow tract aortas. The left atrioventricular valve has two leaflets, anterior and posterior, and is the bicuspid valve or mitral valve. The right atrioventricular valve has a third leaflet (small, septal cusp) and is the tricuspid valve.
(More? Cardiovascular System Development | Lecture - Heart Development | Cardiac Embryology Online Tutorial)

Heath-Edwards classification

(Heath-Edward grade) A pathological grading system for pulmonary artery structural changes that occur with congenital cardiac septal defects. The classification is named after the two original paper authors Donald HEATH and Jessee EDWARDS (PMID 13573570) and grades from I to VI with increasing severity of arterial changes.
  1. Grade I - hypertrophy of the media of small muscular arteries and arterioles.
  2. Grade II - intimal cellular proliferation in addition to medial hypertrophy.
  3. Grade III - advanced medial thickening with hypertrophy and hyperplasia including progressive intimal proliferation and concentric fibrosis. Results in an obliteration of the arterioles and small arteries.
  4. Grade IV - "plexiform lesions" of the muscular pulmonary arteries and arterioles with a plexiform network of capillary-like channels within a dilated segment.
  5. Grade V - complex plexiform, angiomatous and cavernous lesions and hyalinization of intimal fibrosis.
  6. Grade VI - necrotizing arteritis.
(More? Cardiovascular Abnormalities Ventricular Septal Defects | Atrial Septal Defects | PMID 13573570 Circulation)

Heath-Edwards grade

See Heath-Edwards classification


The common blood island progenitor cell which forms both hematopoietic (blood) and endothelial (blood vessel) cells.
(More? Blood Development Blood Vessel Development)


(Greek, hemi = half, melia = limb) limb abnormality with the absence of half a limb, as described in the original classical classification of limb deficiencies.
(More? Limb Abnormalities - Classification)

hemotrophic nutrition

Term used to describe in late placenta development the transfer of blood-borne nutrition from maternal to embryo/fetus compared to early [H#histiotrophic_nutrition|histiotrophic nutrition].
(More? Placenta Notes | Uterine glands provide histiotrophic nutrition for the human fetus during the first trimester of pregnancy. Burton GJ, Watson AL, Hempstock J, Skepper JN, Jauniaux E. J Clin Endocrinol Metab. 2002 Jun;87(6):2954-9. PMID: 12050279 J Clin Endocrinol Metab.)

Hensen's node

(primitive node, primitive knot) See primitive node. forms the initial region at the cranial end of the primitive streak (where gastrulation occurs) and is a controller of this process. It is also the site of epiblast extension cranially to form the initial axial process. Region is equivilant to the blastopore in amphibians.
(More? Carnegie stage 7 | original Carnegie Stage 7 | Week 3 Gastrulation | Neural Notes | Nobel Laureate- Hans Spemann)

heparin-binding epidermal growth factor-like growth factor

(HBEGF) Member of the EGF family synthesized as transmembrane proteins that signal to adjacent cells, appears to be involved in the signaling between the endometrium and implanting trophoblast cells, resulting in a synchronization of their corresponding developmental programs. Expression by trophoblast cells also appears to regulate extravillous differentiation. In mice, has been identified as an important factor required by the uterus for implantation. Expression pattern is different between primates and other species, in the human menstrual cycle late proliferative phase until the early secretory phase HBEGF is mainly localized in the stromal compartment of the endometrium.
(More? Placenta Development | PMID: 19565643 | PMC2789565)


(Greek, hepato = liver) Term used in relation to the liver and its associated structures.
(More? Gastrointestinal Tract - Liver Development | Gastrointestinal Tract Development)

hepatic duct

The liver excretory duct, joins with gall bladder cystic duct to form the common bile duct.
(More? Gastrointestinal Tract - Liver Development | Gastrointestinal Tract Development)


The undifferentiated liver progenitor cell formed initially from endoderm, which willlater form both hepatocytes and biliary cells.
(More? Gastrointestinal Tract - Liver Development | Gastrointestinal Tract Development)

hepatopancreatic ampulla

(ampulla of Vater) The gastrointestinal tract location of bile and pancreatic duct emptying into the duodenum.
Bile Pathway: bile canaliculi → intrahepatic bile ductules (canals of Hering) → interlobular bile ducts → intrahepatic bile ducts → left and right hepatic ducts merge to form → common hepatic duct (exits liver) → joins cystic duct (from gall bladder) forming → common bile duct → joins pancreatic duct → forming hepatopancreatic ampulla (ampulla of Vater) → enters duodenum
(More? Gastrointestinal Tract - Liver Development | Gastrointestinal Tract Development)


The functional liver cell formed from hepatoblast differentiation (hepatoblasts form from endoderm).
(More? Gastrointestinal Tract - Liver Development | Gastrointestinal Tract Development)


A peptide involved in iron homeostasis which is regulated by bone morphogenetic proteins (BMPs), cytokines belonging to the TGF- superfamily.


The commercial name for an antibody that attaches itself to HER2 on breast cancer tumors, inhibiting the tumor's ability to grow. Approximately 15 - 25 % of breast cancer patients have a gene mutation known as HER2-positive.


(Disorder of Sex Development, DSD) This historic terminology is no longer applied to abnormal sexual development and has been replaced with the term (Disorder of Sex Development (DSD). Humans having both male and female reproductive organs, occurs in both male and female forms and mixed ovotesticular DSD.
(More? Genital System - Abnormalities)


A general discription of protrusion of an organ through a weak spot in the surrounding tissue. In normal development, herniated midgut, describes the gastrointestinal tract growth outside the abdominal wall prior to body wall growth. In abnormal development, abnormal protrusion of organs in the diaphragm, abdominal or groin areas (hiatal hernias or inguinal hernias).


The description of the process of protrusion of an organ through a weak spot in the surrounding tissue. In normal development, herniated midgut, describes the gastrointestinal tract growth outside the abdominal wall prior to body wall growth. In abnormal development, abnormal protrusion of organs in the diaphragm, abdominal or groin areas (hiatal hernias or inguinal hernias). Occurs normally in the development of the gastrointestinal tract when the midgut is initially herniated at the umbilicus during embryonic development.

Hertwig's epithelial root sheath

(HERS) A bilayered epithelial structure required for tooth root formation (root organization induction, biomineralization), which also interact with cranial neural crest derived ectomesenchyme to guide root development. The two layers have different embryonic origins; an ectodermal outer and inner enamel epithelium layer. Named after Oskar Hertwig (1849 - 1922) who identified in amphibia in 1874.
(More? tooth | Lecture - Integumentary Development | PMID 19576204)


(Greek heteros = different and taxis = arrangement) is the right/left transposition of thoracic and/or abdominal organs.
(More? Human Abnormal Development)

heterotopic pregnancy

(Greek, heteros = other) Clinical term for a pregnancy of two or more embryos, consisting of both a uterine cavity embryo implantation and an ectopic implantation. The ectopic implantation usually occurring within the uterine tube (tubal pregnancy).
(More? Heterotopic Pregnancy | Ectopic Implantation | implantation)


(Greek, heteros = other + trophe = nourishment) An organism that cannot derive energy from sunlight or from inorganic chemicals but must obtain energy by degrading organic molecules.


Having two different alleles for a single gene (in a diploid organism).


Acronym for the HUGO Gene Nomenclature Committee, the worldwide authority that assigns standardised nomenclature to human genes.
(More? Signaling | HGNC)


(Greek, hiatum = to gape) anatomical description of a gap, cleft or opening.


Acronym for Haemophilus influenzae type b vaccine.
(More? Template:Immunization)


A transcription factor that is one of the main regulators of homeostasis in human tissues exposed to hypoxia, due to inflammation and/or insufficient circulation.
(More? respiratory)

High mobility group box 1 protein

(HMGB1) A protein that regulates the process of endochondral ossification. HMGB1 accumulates in the cytosol of hypertrophic chondrocytes at bone growth plates and is secreted to act as a chemoattractant for osteoclasts, osteoblasts, and endothelial cells. The protein has other functional roles within the cell nucleus.
(More? bone | PMID 1952123)

high risk pregnancy

Term relates to multiple pregnancies, previous pregnancy problem, health problem (either before or due to the pregnancy) and age (over 35 years). Examples of maternal health conditions leading to high risk pregnancy include: autoimmune disorders, cancer, diabetes, heart disease, high blood pressure, kidney problems and sexually transmitted diseases.

Hilgenreiner's line

A clinical x-ray landmark of hip development. The line is formed by a horizontal line drawn between the two triradiate cartilage centers of the hips. It defines a horizontal plane and an approximation to flexion axis of the hips. This is also used to determine the acetabular index in hip development.
(More? Developmental Hip Dysplasia | Musculoskeletal Abnormalities}


Term used to describe an anatomical depression in an organ where vessels and nerves enter or leave.


(rhombencephalon) The common term used to describe the early primary brain vesicle lower subdivision of brain development at the stage when there are three primary vesicles or expansions of the early neural tube (forebrain, midbrain, hindbrain). These will later form the two secondary brain vesicles, the metencephalon and the myeloncephalon. These will in turn generate in the adult brain the cerebellum, pons and medulla. The hindbrain lumen (the cavity of the neural tube) will form the fourth ventricle.
Three primary brain vesicles: forebrain ( prosencephalon) - midbrain (mesencephalon) - hindbrain (rhombencephalon)
(More? neural | Lecture - Early Neural Development)


The last of the three part/division (foregut - Midgut - hindgut) of the early forming gastrointestinal tract. The hindgut forms all the tract from the distral transverse colon to the cloacal membrane and extends into the connecting stalk (placental cord) as the allantois. In addition, a ventral of the hindgut will also form the urinary tract (bladder, urethra) epithelium. These anatomical divisions also correspond to their 3 main vascular supply divisions of foregut coeliac artery, midgut superior mesenteric artery and hindgut inferior mesenteric artery.
Gastrointestinal Tract Divisions: foregut - Midgut - hindgut)
(More? gastrointestinal | renal)

Hirschsprung's Disease

(intestinal aganglionosis, aganglionic colon, megacolon, congenital aganglionic megacolon) Gastrointestinal tract abnormality due to a lack of enteric nervous system (neural ganglia) in the intestinal tract responsible for gastric motility (peristalsis). In general, its severity is dependent upon the amount of the GIT that lacks intrinsic ganglia, due to an earlier developmental lack of neural crest migration into those segments. Historically, Hirschsprung's disease takes its name from Dr Harald Hirschsprung (1830-1916) a Danish pediatrician (of German extraction).
(More? neural crest abnormalities | neural crest | Gastrointestinal Tract Development)


Term used to describe male pattern hair (terminal hair: long, coarse, and pigmented) growth in the female due to androgens. The condition affects 5-15% of women of reproductive age and may be ethnic or genetic, but may also relate to the gonadal abnormality of polycystic ovary syndrome. Androgens from the adrenal glands and ovaries increase hair growth by converting vellus hair (fine, unpigmented) to terminal hair in areas of the face, chest, and abdomen. This condition differs from generalised hair growth not exclusive to androgen sensitive areas (hypertrichosis).
(More? Polycystic Ovary Syndrome | hair)

hip click

Clinical simplified term describing the audible "click" or “pop” sound occurring during postnatal examination of an infant hip, while testing for Developmental Hip Dysplasia. These hip clicks can be absent in newborn infants and can also occur in infants both with or without developmental hip dysplasia.
(More? Developmental Hip Dysplasia | Musculoskeletal Abnormalities}

Hippocratic Oath

A medical oath historically taken by physicians that in its original form, it requires a new physician to swear, by a number of healing gods, to uphold specific ethical standards. Originally written in Greek in the 5th century BC by Hippocrates, who was described as "the father of medicine".
(More? Hippocratic Oath | Hippocrates | 1932 Essays on the History of Embryology)

histiotrophic nutrition

Term used to describe in early placenta development the intital transfer of nutrition from maternal to embryo (histiotrophic nutrition) compared to later blood-borne nutrition (hemotrophic nutrition). Histotroph is the nutritional material accumulated in spaces between the maternal and fetal tissues, derived from the maternal endometrium and the uterine glands. This nutritional material is absorbed by phagocytosis initially by blastocyst trophectoderm and then by trophoblast of the placenta. in later placental development nutrition is by the exchange of blood-borne materials between the maternal and fetal circulations, hemotrophic nutrition. During the embryonic period uterine glands secrete at least 2 glycoproteins (mucin MUC-1 and glycodelin A) that are taken up by the syncytiotrophoblast cells.
(More? placenta | Uterine glands histiotrophic nutrition PMID 12050279)


(Greek, histo = "tissue"; genesis = "a coming into being") Historic term used to describe the study of the embryonic origins of organs and tissues. This was generally a microscopic study of the structural and functional differentiation of these different tissues of the body.
(More? Embryology History | fetal)


(Greek, histo = "tissue" and logia = “to speak”) The study of tissue microscopic structure often using sectioning of the tissue, fixation to preserve structure and stains or dyes to specifically label cellular and extracellular structures.


(Greek, histos = web) One of a set of small, positively charged proteins that bind to DNA in eukaryotic cells. Their role is to "pack" DNA strands into a smaller volume to form the chromosome.
(More? Molecular Development - Genetics)


Acronym for Human Immunodeficiency Virus.


An acronym for histocompatibility antigen or Human Leukocyte Antigen.


(HLA-6.0; HLA60, T-CELL A LOCUS, TCA) An acronym for histocompatibility antigen, class I, G (also called Human Leukocyte Antigen G) and is expressed on placental cytotrophoblast cells and other adult tissues. This distinct tissue distribution differs from the other HLA antigens (HLA-A, HLA-B, HLA-C) leading to the description as a non-classical class I antigen. The molecule is a heterodimer consisting of both a heavy chain and a light chain (beta-2 microglobulin). The heavy chain is anchored in the membrane. Human gene is located at 6p22.1 and there exist several protein isoforms from alternative splicing of messenger RNAs.
(More? trophoblast | placenta | OMIM142871)

Hofbauer cells

Cells found within placental villi connective tissue. Have a role as macrophages of mesenchymal origin with potentially additional functions (remodeling, vasculogenesis, regulation of stromal water content).
(More? placenta)


Early development term referring to complete cleavage of the zygote occurring in amphibian and mammal zygote development. Alternative form of division is described as meroblastic, refers to partial cleavage generating a large yolk, occurring in birds and fish development.
(More? zygote | morula)


A neural development abnormality resulting from incomplete cleavage of the forebrain (prosencephalon). Associated facial abnormalities can include: cyclopia, proboscis, median or bilateral cleft lip/palate (severe forms), ocular hypotelorism or solitary median maxillary central incisor (minor forms). Seven genes have been positively implicated: sonic hedgehog (SHH), Zinc finger protein of the cerebellum 2 (ZIC2), Sine Oculis Homeobox 3 (SIX3), Transforming Growth Factor-beta-induced Factor (TGIF), Patched 1 (PTCH1), Gli-Kruppel family member 2 (GLI2) and TDGF1. In humans, 1 in 250 first-trimester embryos can have this condition, live birth prevalence is 1 in 16,000.
(More? neural | OMIM236100 PMID 17274816)

Holt-Oram syndrome

Congenital development anomaly due to mutations in the TBX5 gene. This autosomal dominant syndrome shows abnormalities of the forearm and hand and associated with secundum atrial septal defect (most common), ventricular septal defect, or rarely, other cardiac malformations. Named after the original describing authors (1960) PMID 14402857.
(More? Cardiovascular Abnormalities Ventricular Septal Defects | Atrial Septal Defects | OMIM142900 | PMID 14402857)


(Greek, homeo = like, similar + stasis = standing) Term used to describe the process of achieving a relatively stable internal environment. Often used in describing the changes required in the fetus to neonate transition following birth (parturition).
(More? Birth | Neonatal Development)

homeotic mutation

A mutation that causes the cells of an embryo to give rise to an inappropriate structure in the adult, for example, to legs instead of antennae.
(More? Fly Development | Homeobox)


A family of structural scaffold proteins expressed in neurons and other cells that also have a role in signaling. In neurons, these proteins have several different locations and functions: in post-synaptic density proteins involved in receptor trafficking and calcium homeostasis; in axonal growth cones involved axon guidance.
(More? Neural System Development | Molecular Development)


Homocysteine is an amino acid homologue of the amino acid cysteine, having an additional methylene (-CH2-) group. It is biosynthesized from methionine and can be recycled into methionine or converted into cysteine with the action of B group vitamins. Folic acid (B9) deficiency can lead to high homocysteine levels and is associated with both neural tube defects and premature birth.

homologous chromosomes

During cell division, the two matching chromosomes that align during meiosis I.
(More? Genetics | Cell Division - Meiosis)


Having two copies of the same allele (in a diploid organism).
(More? Genetics)


(Greek, hormao = "I excite or arouse") A substance, made and released by cells in a specific organ or structure, that moves throughout the organism and exerts specific effects on specific cells in other organs or structures.
(More? Endocrine System Development | Lecture - Endocrine Development)

hormone replacement therapy

(HRT, hormone therapy, HT) Clinical term referring to the treatment of mainly menopausal women to prevent discomfort associated with decreases in circulating estrogen and progesterone hormones. The therapy artificially increases these estrogens, progesterone or progestins, and sometimes testosterone hormone levels. Also used in transgender, either male to female or female to male, treatments.
(More? Menstrual Cycle)

horseshoe kidney

(renal fusion) Term referring to renal development abnormality where the two kidneys form anatomically fused together, typically at their lower poles. The name comes from the appearance of the two fused kidneys being shaped like a horseshoe.
(More? Image - horseshoe kidney | Renal Abnormalities | Renal System Development)

Howship's lacunae

(resorptive bay) The historic 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. Named after John Howship (1781 - 1841) an English surgeon.
(More? Image - Osteoclast | Bone Histology | Lecture - Musculoskeletal Development)

Hox gene

Acronym from homeo box gene identified as a conserved region of protein sequence required for DNA specific binding. Hox proteins are transcription factors regulating gene activity within cells during development and differentiation, have an important role in "patterning" tissues during development. These are the mammalian counterpart of a Drosophila homeo domain gene, for example the antennapedia mutation.
(More? Homeobox | Molecular Development | Fly Development)

Hox B8

(HOMEOBOX B8, HOXB8) Member of the Hox gene family that act as transcriptional regulators during development and in the adult. Knockout mice behave with excessive pathologic grooming behavior, leading to hair removal and self-inflicted wounds at overgroomed sites. Hoxb8 is expressed in regions of the CNS known as the 'obsessive-compulsive-disorder-circuit' where OCD patients are thought to have abnormal metabolic activity.
(More? Homeobox | OMIM - Hox B8)


Acronym for Human Papilloma Virus, a group of more that 100 different viral strains. Detectable by pap smear
(More? Abnormal Development - Viral Infection | CDC STD Facts - Human papillomavirus | Medline Plus Human Papillomavirus | Human Menstrual Cycle - PapSmear)


Clinical acronym for hormone replacement therapy.


Acronym for Haematopoietic Stem Cell, note the alternate US spelling Hematopoietic Stem Cell.
(More? Blood Development)

human chorionic corticotropin

(human chorionic adrenocorticotropin, hCACTH) Placental hormone thought to have corticotropin (ACTH)-like activity, increasing maternal cortisol levels.
(More? Placenta Development | Lecture - Placenta Development | Lecture - Endocrine Development)

human chorionic gonadotrophin

(hCG, human chorionic gonadotropin) UK spelling for human chorionic gonadotropin.
(More? Human Chorionic Gonadotropin)

human chorionic gonadotropin

(hCG) Placental hormone initially secreted by cells (syncitiotrophoblasts) from the implanting conceptus during week two. Functions initially to support the maternal ovarian corpus luteum, which in turn supports the endometrial lining and therefore maintains pregnancy. Hormone can be detected in maternal blood and urine and is the basis of many pregnancy tests. Hormone also stimulates the onset of fetal gonadal steroidogenesis, high levels have been shown to be teratogenic to fetal gonadal tissues.
(More? Human Chorionic Gonadotropin | Endocrine Placenta | Placenta Development | Week 2 | Week 3 | PMID 20735820)

human chorionic somatomammotropin

(hCS, human chorionic somatommotropin, human placental lactogen, HPL) Placental peptide hormone, similar to pituitary growth hormone, secreted by placental syncytiotrophoblasts. Hormone level increases in maternal blood through pregnancy, decreases maternal insulin sensitivity (raising maternal blood glucose levels and decreasing maternal glucose utilization) aiding fetal nutrition. Has some weak growth hormone activity.
(More? Lecture - Placenta Development | Lecture - Endocrine Development | Placenta Development | Mammary Gland)

human chorionic thyrotropin

(hCT) Peptide placental hormone, similar to anterior pituitary released thyroid stimulating hormone (TSH), which along with human chorionic gonadotrophin (hCG) is thought to act on maternal thyroid. There is little recent research published on this hormone, its level and activities.
(More? Lecture - Placenta Development | Lecture - Endocrine Development | Placenta Development | Mammary Gland)

human epididymis protein 4

(HE4, Whey Acidic Protein four-disulfide core domain protein 2, WFDC2) Also found in the blood serum of ovarian cancer patients.
(More? PMID 15781627 PMID 23228410)

Human Immunodeficiency Virus

(HIV) The virus which leads to Acquired Immune Deficiency Syndrome (AIDS), a fatal disease that destroys the immune system's ability to fight off infection. HIV virus is capable of being transmitted from mother to fetus.
(More? Abnormal Development - Viral Infection)

Human induced pluripotent stem cells

(hiPSCs) Human stem cells generated by the de-differentiation of adult somatic cells. The same type of induced cell in other species are referred to as induced pluripotent stem cells (iPSCs) and requires 4 specific genes to be expressed.

human embryo

The term used in describing the first 8 weeks of human development following fertilization or gestational age GA week 10. Historically divided into 23 Carnegie stages and is the time of organogenesis. The following period defined as fetal development. (More? Embryonic Development)
Biological definition (NHMRC 2006):
"human embryo means a discrete entity that has arisen from either:
(a) the first mitotic division when fertilisation of a human oocyte by a human sperm is complete; or
(b) any other process that initiates organised development of a biological entity with a human nuclear genome or altered human nuclear genome that has the potential to develop up to, or beyond, the stage at which the primitive streak appears; and has not yet reached 8 weeks of development since the first mitotic division."
Human Embryo - Biological definition  
Modern Definition

The following biological definition comes from the Australian National Health and Medical Research Council (NHMRC) discussion paper (2006).

"human embryo means a discrete entity that has arisen from either:
(a) the first mitotic division when fertilisation of a human oocyte by a human sperm is complete; or
(b) any other process that initiates organised development of a biological entity with a human nuclear genome or altered human nuclear genome that has the potential to develop up to, or beyond, the stage at which the primitive streak appears;
and has not yet reached 8 weeks of development since the first mitotic division."

This definition was also published later by the same group in 2007.[1]

Historical Definition

"The distinction between the embryonic and the fetal periods at 8 postovulatory weeks has proved valuable. It is based primarily on the probability that more than 90 percent of the more than 4,500 named structures of the adult body have appeared by that time."[2][3]

Links: Embryonic Development | Fertilization | NHMRC paper PDF
(More? Embryonic Development | 2005 NHMRC Discussion paper - biological definition | PMID 17178746)

human menopausal gonadotropin

(HMG) A clinical hormone preparation used in assisted reproductive technologies (ART). This hormone is collected from the urine of menopausal women and has similar biological activity to that of follicle stimulating hormone (FSH). This is used in an injectable form along with human chorionic gonadotropin (hCG) to induce ovulation. Some commercial product names include Menogon or Organon.
(More? Ovarian Stimulation | In Vitro Fertilization | Menstrual Cycle)

human leukocyte antigen

(HLA) human major histocompatibility complex.

Human Papilloma Virus

(HPV) a group of more that 118 identified viral strains about 40 infect the genital tract and 12 are known to be cancer-causing. Just eight HPV types 16, 18, 45, 33, 31, 52, 58, and 35 in descending order of frequency are responsible for more than 90 percent of cervical cancer cases. (Lancet 2010) Infection can be detected by pap smear and at least 2 vaccines Cervarix (GSK) and Gardasil (Merck) protect against HPV types 16 and 18, and through cross-protection also partially against HPV types 31 and 45.
(More? Human Papilloma Virus | Menstrual Cycle - Histology | CDC STD Facts - Human papillomavirus | Medline Plus Human Papillomavirus)

human umbilical cord perivascular cells

Cells that surround the umbilical cord blood vessels and when isolated have been identified as a source of human mesenchymal stem cells. Described as umbilical cord derived mesenchymal progenitors (HUCPVCs) forming single-cell-derived (SCD) clonal populations.

humoral immunity

(Latin, umor = fluid) The production of antibodies by the immune system that recognise foreign material to the body.
(More? Immune System Development)

Hunter Russell syndrome

(Hunter-Russell syndrome) Neural bnormality named after two UK clinicians, Donald Hunter and Dorothy Russell, who initially in 1940 and later in 1954 identified neural abnormalities (focal cerebellar and cerebellar atrophy) in a human subject due to organic mercury compounds PMID 13212411.
Hunter, D.. Bomford, R. R., and Russell, D. S. (1940). Quart. J. Med., 9, 193.
lnnes. J. R. M., Russell, D. S., and Wilsdon, A. J. (1940). J. Path. Bact, 50, 455.
(More? Abnormal Development - Heavy Metals | Neural Abnormalities | PMID 13212411)

Huntington disease

(Huntington's disease, HD) Genetic disorder, autosomal dominant inherited disease caused by an elongated CAG repeat (36 repeats or more) on the short arm of chromosome 4p16.3 in the Huntingtine gene. Mean age at onset of symptoms is 30-50 years, there is no current cure. Prenatal diagnosis is possible by chorionic villus sampling or amniocentesis.
(More? Chorionic villus sampling | Amniocentesis)

Hutchinson-Gilford Progeria Syndrome

(HGPS) a rare genetic disorder characterized by dramatic premature aging. It occurs due to a point mutation in the gene for nuclear lamin A (LA) protein, required for normal nuclear structure, the mutation generates a mutant protein.
(More? PMID 17360355)

Hutchinson's teeth

(Hutchinson's incisor, Hutchinson's sign, Hutchinson-Boeck teeth) Historic clinical term for an infant tooth abnormality associated with congenital syphilis. Teeth are smaller, more widely spaced than normal and have notches on the biting surfaces. Named after Jonathan Hutchinson (1828 – 1913) an English surgeon and pathologist, who first described this association.
(More? Abnormal Development - Syphilis | Tooth Development)

Hv1 proton channel

Membrane channel found on spermatozoa, thought to be involved with the primary activation prior to fertilization.


(hyaluronic acid or hyaluronate) An extracellular matrix glycosaminoglycan disaccharide composed of D-glucuronic acid and D-N-acetylglucosamine. Has a role in embryonic development and in joint fluid, space filling role in resisting compressive forces. In oocyte development, hyaluronan is found in the cumulus extracellular matrix and has a role in the detachment of the cumulus-oocyte complex that becomes freely floating in follicular fluid before ovulation.
(More? Joint Development)

hydantoin syndrome

Term used to describe a fetal disorder caused by exposure of a fetus to the anticonvulsant drug phenytoin (Dilantin) used in the treatment of epilepsy.
(More? Abnormal Development - Drugs | Human Abnormal Development | Medline Plus - phenytoin)


Term used to describe "water".

hydatiform mole

A uterine tumour with "grape-like" placenta appearance without enclosed embryo formation, arises mainly from a haploid sperm fertilizing an egg without a female pronucleus. It is one form of gestational trophoblastic disease (GTD), a number of abnormalities including hydatiform mole, invasive mole, choriocarcinoma and placental site trophoblastic tumor (PSTT).
(More? Hydatidiform Mole | Placenta Development | Week 2 Abnormalities)


(Greek, hydro = water, coele/koilia = cavity) a fluid-filled cavity of either testis or spermatic cord, where peritoneal fluid passes into a patent processus vaginalis.
(More? Testis Development | Genital System Development)


(Greek, hydro = water + cephalus = brain) A developmental abnormality of the rostral neural tube, in development where the anterior neuropore fails to close leading to a large fluid-filled space rather than the normal brain development. In young children, hydrocephalus can often lead to enlargement of the head (skull) as the bones of their cranial vault are not yet fused. Normal-pressure hydrocephalus (NPH) is an adult-onset syndrome involving non-obstructive enlargement of the cerebral ventricles. See also obstructive hydrocephalus.
(More? Neural System Development | Neural System - Abnormalities)


Clinical condition caused by accumulation of fluid in the vagina due to developmental obstruction by either an imperforate hymen or a transverse vaginal septum. Associated complications include: multiple urinary tract infections, hydrocolpos infection, sepsis, failure to thrive, ruptured hydrocolpos, and development of hydronephrosis in previously normal kidneys.
(More? Vagina Development | Genital - Female Development | PMID 20620327)


(Greek, hydro = water + lysis = breaking) Breaking the bond between two building blocks by adding a water molecule, reversing the dehydration-condensation reaction.


(congenital hydronephrosis, Greek, hydro = water) A kidney abnormality due to partial or complete obstruction at the pelvi-ureteric junction. This leads to a grossly dilated renal pelvis causing extensive renal damage before birth. This abnormality may be familial, may be lateral, and is most commonly an intrinsic defect in the wall of the ureter (structural or functional). The less severe cases may be salvaged by reconstruction of the pelvi-ureteric junction.
(More? Renal System - Abnormalities | Renal System Development)


(Greek, hydro = water + phobos = fear) Avoiding associations with water; nonpolar. The type of charge interaction which leads to the bilaminar structure of membranes where the nonpolar molecular "tails" face each other and polar "heads" are exposed to water.

hydrops fetalis

(Greek, hydro = water) accumulation of interstitial fluid (edematous) in the fetus resulting in utero or perinatal morbidity and mortality. The two main forms are immune (Rh incompatibility) or non-immune (disruption of fluid management).
(More? Medline Plus - hydrops fetalis)


(Greek, hydro = water + salpinx = trumpet) Accumulation of interstitial fluid (edematous) in either one or both blocked uterine tubes or fallopian tubes due to a previous tubal infection. (Salpinx refers to the trumpet shape of the uterine tube. This blockage can impact upon maternal fertility and may require in vitro fertilization (IVF) techniques for reproduction.
(More? Uterus Development | ASRM - Hydrosalpinx Patient FactSheets PDF)

hygiene hypothesis

(microbial deprivation hypothesis) A relatively new theory (yet to be tested) that suggests that first world disease like asthma, inflammatory bowel disease, multiple sclerosis, and others may be due in part to lifestyle and environmental changes that make us too "clean" for our environment. The hypothesis is that early exposure to environmental factors directs the body’s immune system along a particular developmental path, and that this in turn offers long-term protection from the development of disease such as asthma. An alternate view/theory is that some microbes can actually enhance antigen sensitization.

hypaxial muscle

(hypomere) Anatomical term describing skeletal muscles which lie ventral (anterior) to the vertebral column developing from the somite myotome. In humans, these are the three body muscle layers and the limb muscles. The muscles dorsal (posterior) to the vertebral column are the epaxial muscles.
(More? Muscle Development | Musculoskeletal System Development)


Bilirubin is produced during red blood cell recycling and processed by the liver to be removed in the stool. Prenatally the placenta removes fetal bilirubin for maternal liver processing. High levels of unconjugated bilirubin (UCB) in the body is associated with a reduction in hepatic UDP glucuronosyltransferase (UGT) 1A1 activity can cause the skin to look yellow, jaundice and can lead to CNS toxicity, brain damage, and even death.
(More? Medline Plus - Newborn jaundice)

hyperemesis gravidarum

(HG) Clinical term for severe form of nausea and vomiting, which are common symptoms of early pregnancy (4 - 16 weeks). Causal factors include increased human chorionic gonadotropin (hCG) and steroids, multiple pregnancy and vitamin deficiency. The condition can lead to dehydration, ketonuria, catabolism and may require hospitalisation.
(More? Human Chorionic Gonadotropin | Maternal liver disease | PMID 22085059)


(heavy periods, excessive menstrual bleeding) Clinical term describing heavy bleeding during the menstrual cycle (menses). This excessive bleeding can be due to a number of different physiological and pathological conditions.
(More? Menstrual Cycle)

hypernatraemic dehydration

A form of newborn dehydration associated with insufficient intake of milk. Blood sodium can increase and if not treated can lead to neurological, circulatory problems and death.


An abnormal increase in organ due to cell proliferation.

hyperplastic rests

In kidney development, embryonic blastema cells can persist and proliferate to form a pool of cells, which under either genetic or epigenetic influence can then change to become a neoplastic rest. Normally the majority of nephrogenic rests either regress or become dormant.
(More? Wilm's tumour | nephrogenic rest | Renal System Development | Renal System - Abnormalities)

hypertension in pregnancy

(preeclampsia) Classified into 4 clinical categories: gestational hypertension (pregnancy induced hypertension after 20 weeks of gestation), chronic hypertension (pre-gestational hypertension of any cause), pre-eclampsia/eclampsia, and chronic hypertension with superimposed pre-eclampsia. Usually defined as a blood pressure of 140/90 mm Hg or above and proteinuria as a daily protein excretion of 300 mg or above, a protein:creatinine ratio of 0.3 or above, or 1+ on a urine dipstick. Pre-eclampsia has also been defined as hypertension combined with proteinuria, but has also been identified recently as occurring without proteinuria.
(More? ACOG - Task Force and Work Group ReportsHypertension in Pregnancy)


Term used to describe a high core body temperature. This has been shown in animal models to be a potent teratogen.
(More? Hyperthermia)


Clinical term describing an overactive thyroid gland.
(More? Endocrine - Thyroid Development | Endocrine System Development)


(Greek hyper = "excess" + troph = "nourishment") A term referring to the increase in size of a cell, tissue or organ due to an increase in volume of each cellular component rather than proliferation of the cells (hyperplasia). This can be part of a normal physiological process or disease condition. Smooth muscle hypertrophy occurs in the uterus during pregnancy. Skeletal muscle hypertrophy occurs after strength training exercises. Cardiac muscle (ventricular hypertrophy) occurs normally as part of exercise or abnormally following increased load (high blood pressure).


(Greek, hypo = beneath) The transient epithelium that forms during week 2 of human development and lines beneath the epiblast layer, these layers together form the bilaminar embryo (epiblast/hypoblast) from the inner cell mass. During trilaminar embryo development, beginning in humans in week 3, the process of gastrulation replaces the hypoblast layer within the embryo with endoderm germ layer.
(More? Week 2 | Carnegie stage 5 | Carnegie stage 6 | Gastrulation)


Clinical term describing mild skeletal dysplasia, due to a mutation in gene encoding fibroblast growth factor receptor-3 (FGFR3), the same gene mutated in the more severe limb Achondroplasia. Genetically heterogeneous, with some cases caused by mutation in genes other than FGFR3.
(More? Musculoskeletal System - Abnormalities | OMIM Hypochondroplasia | FGFR3)


(subnotochordal rod) An endodermal derived transient rod-like structure formed by a a row of single cells that lies immediately ventral to the notochord in the amphibian and fish embryo. Has a role in patterning the development of the dorsal aorta.
(More? Endoderm | Frog Development | Zebrafish Development | PMID 10648245)


Term describing a congenital absence of teeth.
(More? Integumentary System - Tooth Development)


Term describing a diminished sweat gland function, and therefore decreased or lack of perspiration.
(More? Integumentary System - Gland Development)

hypohidrotic ectodermal dysplasia

(HED) An abnormality of skin characterised by: sparseness of scalp and body hair, reduced ability to sweat and congenital absence of teeth. Occurs in three distinct forms: X-linked recessive (EDA gene), autosomal recessive forms (EDAR and EDARADD genes) and autosomal dominant form (EDAR and EDARADD genes).
(More? Integumentary System - Gland Development | GeneReviews - Hypohidrotic Ectodermal Dysplasia)
hypopharyngeal eminence
hypopharyngeal eminence

hypopharyngeal eminence

(hypobranchial eminence) An early embryonic structure in the developing head. A narrow midline mesoderm (mesenchymal) exension lying within the floor curve of the developing pharynx. Fusion of 3rd pharyngeal arches and precursor of root of tongue. Early developing thyroid cells also migrate into this structure as cords of cells.
(More? Head Development | Endocrine - Thyroid Development)


Clinical term for a decreased amount of of scalp and body hair.
(More? Integumentary System Development | Hair Development)


(pituitary gland) 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)


A male external genital abnormality resulting from a failure of male urogenital folds to fuse in various regions and are therefore classified by the location of the opening (meatus). This is the most common penis abnormality (1 in 300) and one of the highest in the list of frequently reported birth defects.
(More? Genital Abnormality - Hypospadia | Genital Abnormalities | Genital System Development | Human Abnormal Development)

hypoxic ischemic encephalopathy

(HIE) Abnormality often occurring with perinatal asphyxia, deprivation of oxygen (hypoxia) and associated restriction in blood supply to tissues (schema) resulting in brain damage. Statistically affecting 1 to 3 per 1000 term births with high mortality and subsequent neural disabilities (10-15% lethal, 15% develop cerebral palsy and many more with developmental delay, cognitive problems, deafness and epilepsy).
(More? Birth)

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 18) Embryology H. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/H

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  1. Findlay JK, Gear ML, Illingworth PJ, Junk SM, Kay G, Mackerras AH, Pope A, Rothenfluh HS & Wilton L. (2007). Human embryo: a biological definition. Hum. Reprod. , 22, 905-11. PMID: 17178746 DOI.
  2. O'Rahilly R. 1979. Early human development and the chief sources of information on staged human embryos. Europ. J. Obstet. Gynec. Reprod. Biol., 9, 273-280. PMID 400868
  3. O'Rahilly R. and Müller F. Developmental Stages in Human Embryos. Contrib. Embryol., Carnegie Inst. Wash. 637 (1987).