UNSW Embryology

Beginnings, Growth and Development

Practical Sexual Differentiation - 3

© Dr Mark Hill (2009)

Acknowledgements

3. Late Embryo (Week 8 stage 22, male)

This page shows small excerpts from whole cross-sections of the late embryonic Carneigie stage 22 human. Read the description with the serial section excerpt and then use the link below each image (click section number) to see the full cross-section image. They are organised in sequence as if you were travelling downward through the embryo (that is why kidney comes first).

Page Links: Introduction | Urinary | Genital | Genital Selected | Sections Table | Glossary | Link to Next Page

Large urogenital movie in new window

Colour Coding Mesonephric Ducts Ureters Mesonephros Gonads Adrenal Glands Kidneys Paramesonephric Ducts Allantois & Bladder Urethra

Urinary System Development

The adult kidneys – the metanephroi – form from day 35, from a portion of the intermediate mesoderm called the metanephric blastema (or metanephric mesenchyme). They are induced to form by the ureteric buds, outgrowths from the end of the mesonephric ducts, which come into contact with the metanephric blastema. Upon contact, they begin to lengthen and bifurcate rapidly in the metanephric blastema – these branches differentiate into the collecting ducts. Both the ureteric buds and the metanephric blastema begin to differentiate; interestingly each induces differentiation in the other structure. The ureteric bud is induced by the metanephric blastema to form the collecting tubules, renal pelvis and ureters. The metanephric blastema is induced to form the nephrons.

Development of the kidney is starts in week 5 and is completed by week 15. However, in week 6 the kidneys begin to ascend to their correct anatomical position. This movement is completed by week 9. During the ascent, the kidneys also become vascularised via the dorsal aorta. As this ascent occurs, the mesonephric ducts and the ureters enter the wall of the eventual bladder.

Simultaneously, between weeks 4 and 6, the cloaca is partitioned into the urogenital sinus anteriorly, and the rectum posteriorly. This is achieved by downward growth of the urorectal septum, a portion of endoderm from the hindgut. The urogenital sinus has an area of enlargement – the bladder – and is superiorly continuous with the allantois.

Further development of the urinary system varies depending on the sex of the embryo. In males, the pelvic urethra forms the membranous urethra, the prostatic urethra and penile urethra; in females it forms the membranous urethra and the vestibule of the vagina. The sex of the above model is male.

Genital System Development

Till the end of the 6th week the male and female genital systems are indistinguishable. Sex differentiation is based upon the presence of specific sex chromosomes. The female has two X chromosomes, while the male has an X and a Y chromosome. The specific gene involved in determining the male sex is the SRY transcription factor, which activates specific genes for male sex development. If SRY is damaged or absent, female development occurs.

In the 5th week, the primordial germ cells migrate to the region of the future gonads. Cells from the coelomic epithelium and the mesonephros proliferate, forming genital ridges medial to the mesonephros. In the 6th week, these cells surround the germ cells, together forming the primitive sex cords. They contain distinct cortical and medullary regions. Also in the 6th week, the paramesonephric or müllerian ducts form, lateral to the mesonephric ducts.

In male genital development, the medullary cells of the sex cords differentiate into Sertoli cells, while the cortical cells degenerate. These primitive cells also secrete anti müllerian hormone (AMH), which causes regression of the paramesonephric ducts between the 8th and 10th weeks. In weeks 9-10, the gonadal cells also begin to produce testosterone, which maintains the mesonephric ducts. These eventually go on to form the vas deferens, ductuli efferentes and other structures in the male.

In the female, the opposite occurs. The sex cords degenerate and the genital ridge forms secondary cortical sex cords. These induce the primordial germ cells to form the ovarian follicles. Due to the lack of AMH and testosterone, the mesonephric ducts degenerate, and the paramesonephric ducts go on to form the fallopian tubes, the uterus and superior part of the vagina. In both sexes, the external genitalia appear similar, until the 12th week.

Region from section (F7 lower left) showing details of the genital duct systems (G2). Mesonephric duct forming the ductus deferens. Paramesonephric duct degenerating in the male. Mesonephric tubules contribute the ductuli efferentes. Testes developing attached by mesorchium (urogenital mesentery) to the mesonephros and ducts.

Do not spend too much time on the kidney image set, go though the genital set in detail. Finally, look at the selected high power images from this same stage, which demonstrate detailed structural relationships within the system.

Trigone

This looped animation shows the posterior of the developing bladder between Week 4 and 6. The mesonephric duct (purple) has lateral branches forming the uteric bud (kidney) both these fuse into the wall of the bladder (yellow). The mesonephric duct then moves inferiorly to the level of the pelvic urethra. Bladder - Trigone

Late Embryo Male (Week 8, Stage 22)

Urinary System

E6: R,L adrenal glands under diaphragm. E7: Large adrenal glands. Inferior vena cava. Thoracic aorta.
	Fl: Adrenal glands. R. Kidney. Autonomic ganglia (partly the adrenal medulla precursors).	F2: Kidneys (note retroperitoneal location). Cortex. Medulla. L. Adrenal gland. Superior mesenteric artery. Inferior vena cava.	F3: R testis (note its location relative to the R adrenal). L adrenal. R renal hilus. large channels are branches of ureteric tree.
F4: R kidney and R ureter. Inferior vena cava. L. kidney, L renal hilus and L ureter. R testis with R mesonephric duct (precursor of vas deferens). L testis. Umbilical arteries passing into umbilical cord allantois between them.	F5: Kidneys. Ureters. Note umbilical arteries and allantois. Also note how R testis and mesonephric structures are attached to parietal peritoneum by a mesogonad.	F6: Kidneys. Ureters. Note umbilical arteries and allantois. Also note how R testis and mesonephric structures are attached to parietal peritoneum by a mesogonad.	F7: In F7, (dorsal to R testis and liver) note with the distinct lumen of the mesonephric duct, almost solid column of paramesonephric cells and remnants of mesonephric tubules. "mesogonad". Ureters. Bladder with submucosa and detrusor muscle. Umbilical arteries. Division of aorta.
G1: Ureters, Bladder. Umbilical arteries. Testis with remains of mesonephros (dorsal), mesonephric duct and paramesonephric cells. Sigmoid colon and mesocolon.	G2: Ureters being displaced ventrally, crossing common iliac arteries. Sigmoid colon. Bladder. Mesonephric ducts (lateral) and paramesonephric ducts (smaller, medial) located dorsal to bladder.	G3: Ureters (cut twice): descending dorsal to bladder and ascending ventrally to enter the bladder at trigone, through the submucosa). Fusion of paramesonephric ducts. Paired mesonephric ducts. Umbilical arteries looping off common iliac arteries. Pubic symphysis. Colon.	G4: Most caudal part of loop of ureters. Urethra emerging from bladder. Mesonephric ducts. Rectocolic junction.
G5: Urethra (in region of future prostate gland - note crescentic shape). Rectum. Rectovesical pouch. Between G4 and G5, each mesonephric duct (vas deferens) has joined the prostatic urethra (caudal to the ureters), thereby increasing the caliber of the latter.	G6: Penile urethra, emerging inferiorly to the glans penis. Scrotal swellings (appear before testis descends).	G7: Penile urethra, emerging inferiorly to the glans penis. Scrotal swellings (appear before testis descends).

Genital System

F3: R testis with thick tunica albuginea (pale) and thin urogenital mesentery.Note spleen on L.	F4: Testes. Narrow urogenital mesentery. Remains of mesonephric tubules near hilus (future efferent ductules at mediastinum testis).	F5: Broader urogenital mesentery on R (still narrow on L).	F6: On R: Remnants of mesonephric tubules (primordia of ductuli efferentes). Mesonephric duct (origin of vas deferens). Laterally, the paramesonephric duct (primordia of appendix testis). Note abdominal location of testes in relation to adjacent structures.
F7: Developing efferent ductules, mesonephric duct and medially, the paramesonephric duct (appearing solid). Note how the paramesonephric duct crossed the mesonephric duct ventromedially. Urogenital mesentery - note how testis is attached to the developing efferent ductules by a separate narrow flange,the mesorchium.	G1: On R: Lateral mesonephric duct and medial paramesonephric groove. (Changing from a tube to a groove). On L: testis (lower than on R), mesonephric tubules, mesonephric duct (lateral), paramesonephric duct (ventromedial - appearing solid).Bladder. Umbilical arteries. Sigmoid colon and mesocolon.	G2: Transition from abdominal cavity to superior pelvis. note iliac crest, femoral nerve, psoas major muscle, internal iliac vein and artery. Medial convergence of paramesonephric cords and more laterally-located mesonephric ducts, ventral to the colon and dorsal to the bladder. Note position of ureters. N.B. Testis lies above this level which is close to the plane of the future inguinal canal.	G3: Transition from superior (major) pelvis to inferior (minor) pelvis. Rectovesical pouch. Note wide expanse of loose tissue ventral to the pubic symphysis: site of testicular descent.
G4: Transition from superior (major) pelvis to inferior (minor) pelvis. Rectovesical pouch. Note wide expanse of loose tissue ventral to the pubic symphysis: site of testicular descent.	External genitalia (male) G5: Minor pelvis. Obturator foramen. Ischium. Head of femur. Acetabulum. Ganglion cells of pelvis plexus. Obturator externus muscle. Urogenital sinus (the caudal enlargement is due to junction of mesonephric ducts).	G6:Level of pelvic floor. Pelvic plexus. Recto-anal junction. Ischium. obturator internus muscle. Greater sciatic notch. Alar of sacrum. Sacro-iliac joint.	G7: Urethra opening ventrally. Genital tubercle (embryonic glans penis).

Genital Human Embryo (stage 22) Selected

	F7 G2: Paramesonephric duct (regressing in male). Mesonephric duct. Mesonephric tubules. Mesorchium. Urogenital mesentery.
	G3: Testis with radiating cell sheets becoming cords (defines the gonad as the testis). Mesonephric tubules (ductuli efferentes). Mesonephric glomeruli. Mesonephric duct - (unlabelled duct to far right) more laterally the thickened coelomic epithelium of the paramesonephric groove. Mesorchium - narrow and pale external layer of tunica albuginea. Metanephros - the definitive kidney.
	G4: Cell plates of testis (becoming cords) containing the germ cells. interstitial cells. large cells with dark nuclei and distinct halo of unstained cytoplasm are the germ cells (spermatogonia).
	G5: Blue box is region shown in detail in the rotated 90 degrees images below (click the images to open full size). Note the anatomical relationship between the now 3 separate systems: gastrointestinal tract (rectum) , urinary (bladder, ureters) and reproductive (mesonephric duct). G6 G7 Fusion of paramesonephric ducts (male - degenerating to form prostatis utriculus). Mesonephric ducts. Ureters. Bladder. Umbilical arteries.

Stage 22 Embryo (Human)

A1	A2	A3	A4	A5	A6	A7
B1	B2	B3	B4	B5	B6	B7
C1	C2	C3	C4	C5	C6	C7
D1	D2	D3	D4	D5	D6	D7
E1	E2	E3	E4	E5	E6	E7
F1	F2	F3	F4	F5	F6	F7
G1	G2	G3	G4	G5	G6	G7

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Glossary

Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.

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Terms

abdomen - trunk between diaphragm and pelvis.

AC - Acronym for Abdominal Circumference.

adenohypophysis - (anterior pituitary, pars distalis) The anterior part of the pituitary, which develops in the early embryo from a transient region on the roof of the pharynx called Rathke's pouch. (More? Endocrine Development - Pituitary)

adnexa - (Latin, adnexae = appendages) Term used to describe any anatomical appendage (accessory structure, extension or outgrowth from the body). In reproductive anatomy used to describe appendages of the uterus "body"; ovaries, uterine tubes and uterus supporting ligaments.

adrenal gland - (suprarenal gland) The endocrine organ that anatomically sits on top of the kidneys (renal). It has two different embryonic origins, neurat crest (aderenal medulla) and mesoderm (adrenal cortex). (More? Endocrine Notes - Adrenal Gland)

adventitia - Anatomical term describing the outermost connective tissue covering of any organ, vessel, or other structure not covered by a serosa. The covering is from the surrounding connective tissue and does not form an integral part of such organ or structure.

allantois - An extraembryonic membrane, endoderm in origin extension from the early hindgut, then cloaca into the connecting stalk of placental animals, connected to the superior end of developing bladder. In reptiles and birds, acts as a reservoir for wastes and mediates gas exchange. In mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)

amnion - An extraembryonic membrane ectoderm and extraembryonic mesoderm in origin and forms the innermost fetal membrane, produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membane led to the description of reptiles, bird, and mammals as amniotes. (More? Placenta Notes | Week 2 Notes)

amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.

ampulla - Term used to describe an anatomical dilation of a tube or canal lumen. Anatomical description of the opening end of the uterine tube lying above the ovary and the enlarged initial segmeny of the semicircular canals of the inner ear vestibular system. (More? Inner Ear | Genital System - Female Uterus)

anastomosis - Term used to describe the connection between two tubes. Applied to describe the connection between peripheral blood vessels without an intervening capillary bed.

androgens - The male sex hormones, eg testosterone. (More? Genital System - Male)

anterior - Anatomical term used to describe the front or ventral surface.

Anti-Mullerian Hormone - (AMH, Mullerian Inhibiting Substance, MIS) A secreted factor (transforming growth factor-beta, TGF-beta superfamily) that regulates gonadal and genital tract development. Inhibits paramesonephric (Mullerian) duct development in males. (More? Male | OMIM - AMH)

atresia - (Greek, a = without + tresis = perforation) Term used for anatomical closing or absence of a cavity or opening that should exist. Used as an antomical, pathological and clinical term: esophageal atresia, biliary atresia, duodenal atresia, jejunal atresia, choanal atresia, vaginal atresia, urethral atresia, pulmonary atresia, bronchial atresia, tricuspid atresia. (More? see atretic follicle)

autosomal - The term decribing all the chromosomes that contribute to a cell's genetic material, except for the sex chromosomes X, Y. (More? DNA Notes)

autosomal inheritance - Some hereditary diseases are described as autosomal which means that the disease is due to a DNA error in one of the 22 pairs that are not sex chromosomes. Both boys and girls can then inherit this error. If the error is in a sex chromosome, the inheritance is said to be sex-linked. (More? DNA Notes)

Carnegie stages - Carnegie stages are a system of classifying embryonic development based on the external features and related internal changes that affect appearance and growth of the embryo. Note that the stages are not directly dependent on either age or size, but upon the appearance of specific embryonic features. Early human and other species embryos can be classified by these stages. The term "carnegie stages" are named after the famous USA Institute which began collecting and classifying embryos in the early 1900's. (More? Carnegie Stages | Carnegie Stages - Scanning Electron Micrography | About the Carnegie Institute | Carnegie Stage Comparision)

caudal - (Latin, caudal = tail) Anatomical term referring to structures that are more towards the tail.

ciliated epithelium - (Latin, cilium = eyelid) An epithelium named on the basis of the cells having surface hair-like appearance of a cilium; singular, cilium. In many tissues, cilia are found as epithelial cell apical surface motile specializations. In the uterine tube epithelium, after ovulation used to move the unfertilized egg, then the fertilized zygote, then blastocyst during the first week of development.

cloacal membrane - Forms the external lower membrane limit (caudal end) of the early gastrointestinal tract (GIT). This membrane is formed during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane breaks down to form the initial "anal opening" of the gastrointestinal tract. (More? buccopharyngeal membrane | GIT Notes)

coelom - Term used to describe a space. There are extraembryonic and intraembryonic coeloms that form during vertebrate development. The single intraembryonic coelom will form the 3 major body cavities: pleural, pericardial and peritoneal. (More? Coelom Notes)

congenital - Already present at birth, often used to describe defects present at birth, congenital defects. (More? Abnormal Development)

corticosteroid - A steroidal hormone produced by the adrenal cortex. (More? Endocrine Notes)

critical period - (critical period of development) The term used to describe a developmental time when exposure to a teratogen can lead to a developmental abnormality, which can be further divided into an early major and later minor developmental abnormality. The defined critical period will differ in timing and length for different systems. (More? Abnormal Development - Environmental)

CRL - Acronym for Crown-Rump Length. Used in embryology to accurately stage the early embryo. Used in clinical ultrasound as a measurement between the periods of 7 to 13 weeks as an accurate estimation of the gestational age. (More? Embryo Stages | Ultrasound Movies)

DAX1 - Acronym for Dosage sensitive sex reversal (DSS), Adrenal hypoplasia congenita (AHC) critical region on the X chromosome, gene 1 , (gene NR0B1) is a nuclear hormone receptor involved in female ovary development. (More? Urogenital Notes | OMIM Entry DAX1)

DAZL - Acronym for DAZ-like due to homology to DAZ (Deleted in AZoospermia), a gene on the long arm of the Y chromosome that is frequently deleted in infertile men with nonobstructive azoospermia. (More? Spermatogenesis | OMIM Entry DAZL)

dihydrotestosterone - The hormonally active form of testosterone (male sex hormone) produced by enzyme (5-alpha reductase) conversion. In the male embryo, this can occur in the genital skin which then supports external genital development. In the adult, this conversion occurs in a number of different tissues. A known treatment for prostate cancer include 5-alpha reductase inhibitors. (More? Urogenital Notes | Endocrine Notes )

ductuli efferentes - testis (male gonad) series of tubular structures which arise from the rete testis and conduct spermatazoa into the ductus epididymidis. Their columnar epithelium lining consisting of both absorptive and ciliated cells (giving rise to "cogwheel appearance) which removes much of the fluid associated with the spermatazoa leaving the testes (also by the upper epididymis) thereby increasing the spermatazoa concentration. (Spermatozoa Duct Pathway: seminiferous tubule → straight tubule → rete testis → ductuli efferentes → ductus epididymidis → ductus deferens) (More? Spermatogenesis | Genital - Male | Genital Notes)

ductus epididymidis - (epididymidis) male testes tubular structure which arise from the ductuli efferentes and conduct spermatazoa into the ductus deferens (vas deferens). The long duct is lined by a tall pseudostratified columnar epithelium. (More? Genital - Male | Genital Notes)

ectoderm - (Greek, ecto = outside + derma = skin) One of the initial 3 germ cell layers, which will form the nervous system from the neural tube and neural crest and also generates the entire epithelial layer of the skin covering the embryo. (More? Week 3 Notes)

embryology - (Greek, en = in + bryein = to be full of) The science of studying embryo development, usually applied to all development before birth (in humans, included both the embryonic and fetal period). (More? UNSW Embryology | embryo stages)

endocrine - (Greek, endon = within) Glands which release hormones into the blood stream. There are many specialized organs and tissues that release hormones into the bloodstream. (More? Endocrine Notes)

endocrine gland - (Greek, endon = within) A gland (organ, tissue) that is specialized for secretion of a hormone into the bloodstream for general circulation. (More? Endocrine Notes)

endoderm - (Greek, endo = inside + derma = skin) One of the initial 3 germ cell layers, formed by the process of gastrulation. The endoderm forms as a cuboidal epithelium and contributes not only to the trilaminar embryo, but also lines the yolk sac. It will form the entire epithelial lining of the gastrointestinal tract (GIT), contribute to the accessory organs of GIT and also forms the epithelial lining of the respiratory tract. Note that in the GIT it contributes both epithelium and the associated epithelial glands. In humans, endoderm forms during week 3 of development. (More? Week 3 - Gastrulation | Week 3 Notes)

epiblast - (Greek, epi = above, upon) the layer (of the bilaminar embryo) that generates endoderm and mesoderm by migration of cells through the primitive streak. The remaing cells form ectoderm.

epithelium - (Greek, epi = upon + thele = nipple) Cells tightly linked together to form a sheet with little extracellular matrix. Most epithelia (plural) in the body are embryonically derived from ectoderm or endoderm germ layers. Note: not "skin" which is the epithelium and includes the underlying connective tissue layers (mesoderm) and melanocytes (neural crest) forming a complex tissue.

epoophoron - (rete ovarii, broad ligament cyst) A group of epithelial tubules that can be located in the mesosalpinx possibly mesonephric in origin. Occurs when a segment of the mesonephric duct remains in the female, associated with either the ovary and broad ligament. This "male remnant" will appear as a cyst (broad ligament cyst, adnexal papillary cystadenoma of probable mesonephric origin, APMO) with an appearance that differs depending upon the state of differentiation when the original abnormality occurred. (More? Urogenital Notes | Genital Notes)

estrogens - Sex hormone found in both male and female. In the female, this hormone is produced by the ovaries and is responsible for development of secondary feminine sex characteristics. Together with progesterone these hormones also regulate changes that occur each menstral cycle. In the male, Leydig cells produce estrogen into the rete testis fluid at variable levels in different species. During male embryonic development exposure to high levels of estrogen can lead to genital abnormalities. (More? Human Menstrual Cycle)

extraembryonic membrane - Term used to describe each of the amnion, yolk sac, allantois and chorion membranes. Amniotic membrane, ectoderm origin innermost membrane, produces amniotic fluid (reptiles, bird, and mammals are amniotes). Yolk sac, endoderm origin, associated with nutrition in reptiles and birds (mammals source of primordial germ cells and blood cells). Allantois, endoderm origin, in reptiles and birds acts as a reservoir for wastes and mediates gas exchange; in mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface. Chorioic membrane, mesoderm origin, outermost layer in reptiles and birds acts in gas exchange; in mammals incorporated into the placenta and its functions. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)

extraembryonic mesoderm - Cells from the conceptus that contribute to placenta and fetal membranes. Described as "extraembryonic" because it is tissue lying outside the embryonic trilaminar disc (ectoderm, mesoderm and endoderm) and "mesoderm", because of the connective tissue cellular organization. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)

first trimester - Clinical term used to describe and divide human pregnancy period (9 months) into three equal parts of approximately three calendar months. The first trimester corresponds approximately to embryonic development (week 1 to 8) of organogenesis and early fetal. The second and third trimester correspond to the fetal period of growth in size (second trimester) and weight (third trimester), as well as continued differentiation of existing organs and tissues. (More? Embryo Stages | Human Fetal Period | Development Week by Week)

follicle stimulating hormone - (FSH, gonadotropin) Glycoprotein hormone secreted by anterior pituitary and acts on gametogenesis and other systems in both males and females. In females, FSH acts on the ovary to stimulate follicle development. Negative feedback by inhibin from the developing follicle decreases FSH secretion. In males, acts on the testis Sertoli cells to increase androgen-binding protein (ABP) that binds androgens and has a role in spermatogenesis. FSH-deficientcy in females results in infertile (block in folliculogenesis prior to antral follicle formation) and in males does not affect fertility (have small testes but are fertile). FSH protein has a molecular weight 30 kDa and a 3-4 hour half-life in circulation. Gonadotrophins have been used clinically in humans for the treatment of infertility. Other glycoproetin hormones include luteinizing hormone (LH), thyroid stimulating hormone (TSH), and chorionic gonadotropin. (More? Week 1 - Oogenesis | Human Menstrual Cycle | Endocrine Development - Pituitary | Kumar TR, Wang Y, Lu N, Matzuk MM. Follicle stimulating hormone is required for ovarian follicle maturation but not male fertility. Nat Genet. 1997 Feb;15(2):201-4.)

gametes - (Greek, gamos = marriage) A specialized reproductive cell through which sexually reproducing parents pass chromosomes to their offspring; a sperm or an egg. (More? Week 1 Notes)

gameteogenesis - The production of either the haploid germ cells of spermatazoa (male) or eggs (female) (More? Week 1 Notes)

Gartner's duct - a female developmental abnormality caused by the persistance of the mesonephric duct (normally lost in females) when the ureteric bud fails to separate from the mesonephric duct. Can generate a broad ligament or vaginal cyst. Named after Hermann Treschow Gartner (1785-1827) a Danish surgeon and anatomist. (More? Genital Abnormalities)

genitalia - (Latin, genitalia = ) The term used to describe either the external or internal male and female sexual and reproductive organs. (More? Urogenital Notes)

genital ridge - (= gonadal ridge) The thickened epithelial/mesenchymal region adjacent and medial too the mesonephros. Primordial sex cells migrate into this region to form the indifferent gonad. These undifferentiated gonads have a cortex and a medulla. Female XX chromosome complex, cortex differentiates into an ovary, and medulla regresses. Male XY complex, medulla differentiates into a testis and cortex regresses. (More? Urogenital Notes)

genital tubercle - A prominence or rounded protuberance extending ventrally at the inferior end of the body of the embryo. It has initially a sexually indifferent external genitalia structure and contributes to either male (glans penis) and female (clitoris) external genitalia. (More? Urogenital Notes)

genome - The collection of all the DNA in an organism. (More? DNA Notes)

germ layers - The first three cellular layers (ectoderm, mesoderm, and endoderm) that will form all tissues of the embryo. In humans, these layers begin to form during week 3 of development. Term should not be confused with germ cells, which are the oocyte and spermatazoa forming cells. Named originally by Robert Remak (1815 - 1865) a German scientist and embryologist. (More? Week 2 Notes Week 3 Notes | Robert Remak)

germinal epithelium - cellular component covering surface of ovary, it is continuous with mesothelium covering mesovarium. Note that it is a historical misnomer, as it is not the actual site of germ cell formation.

GHRH - Arconym for Growth Hormone Releasing Hormone, secreted by the Hypothalamus it is a protein that activates Growth Hormone synthesis and release from the pituitary. (More? Endocrine Notes - Hypothalamus)

Globozoospermia - also called round-headed spermatozoa, is a human infertility syndrome caused by spermatogenesis defects leading to a malformed or absent sprematazoa acrosome. The acrosomal reaction being a key component of sperm fertilization of the egg. (More? Week 1 Abnormalities | OMIM - Globozoospermia)

gonad - (Greek, gonos = seed) A gamete-producing (germ cell) organ. A non-sexual term which is used to describe both the female ovary and male testis. (More? Week 1 Notes)

gonadotrophin releasing hormone - (Greek, gonos = seed) (GnRH) Hormone released from hypothalamus that stimulates pituitary gonadotropin synthesis and secretion (luteinizing hormone, LH and follicle stimulating hormone, FSH). The cyclic release of GnRH has been shown to differentially affect gonadotropin release (rapid frequency, more than 1 pulse / hour LH; slower frequencies FSH secretion). (More? Week 1 Notes | Marshall JC, Eagleson CA, McCartney CR. Hypothalamic dysfunction. Mol Cell Endocrinol. 2001 Oct 25;183(1-2):29-32. Review.)

granulosa cell - A specific cell type that proliferates in association with the oocyte within the developing follicles of the ovary. These cells form the follicle stratum granulosa and are also given specific names based upon their position within the follicle. In the antral follicle, membrana granulosa sits on the follicular basal lamina and lines the antrum as a stratified epithelium. The cumulus oophorus is a column of granulosa cells that attaches the oocyte to the follicle wall. The corona radiata are the granulosa cells that directly surround the oocyte, and are released along with it at ovulation. Following ovulation the corona radiata provide physical protection to the oocyte and granulosa cells within the ovulating follicle contribute to corpus luteum. (More? Week 1 - Oogenesis | Corpus luteum)

growth hormone - (GH) A peptide hormone, made in the anterior pituitary, that stimulates tissue and skeletal growth. (More? Endocrine Notes - Pituitary)

growth hormone releasing hormone - (GHRH) secreted by the hypothalamus it is a protein that activates Growth Hormone synthesis and release from the pituitary. (More? Endocrine Notes - Hypothalamus)

gynecomastia - (Greek, gyne = woman, mastos = breast) is the excessive development of the male breast, which can occur transiently in puberty or due to other (hormonal) abnormalities. (More? Integumentary Development - Mammary Glands)

hCG - An acronym for the hormone human Chorionic Gonadotrophin.

hermaphrodite - Term used for having both male and female reproductive organs, in humans there are three general forms of this abnormality.

1. True Hermaphrodites (46,XX) karyotype with gonads containing both ovary and teste tissues, Ovotestes or ovary and testes.
2. Male Pseudohermaphrodites (46,XY) 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.
3. Female Pseudohermaphrodites (46,XX) karyotype with gonads as ovaries, external genitalia ambiguous and hyperplastic adrenals secreting androgens.

(More? Reproductive System - Abnormalities)

hernia - 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).

herniated - The discription 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.

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

hindgut - 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. (More? Gastrointestinal Tract Notes | Urogenital Notes)

hormone - 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 Notes)

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

human chorionic gonadotrophin - (hCG) Placental hormone initially secreted by cells (syncitiotrophoblasts) from the implanting conceptus during week two, supporting the 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 teh basis of many pregnancy tests. Hormone also stimulates the onset of fetal gonadal steroidogenesis, high levels are teratogenic to fetal gonadal tissues. (More? Placenta Notes | Week 2 Notes)

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? Placenta Notes | Integumentary Development - Mammary Glands)

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? Placenta Notes | Integumentary Development - Mammary Glands)

hydronephrosis - (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? Urogenital Abnormalities)

hyperplasia - An abnormal increase in organ due to cell proliferation.

hypospadia - A male external genital abnormality, which is the most common penis abnormality (1 in 300) resulting from a failure of male urogenital folds to fuse in various regions and are therefore classified by the location of the opening (meatus). (More? Genital Abnormalities - Hypospadia | Genital Notes)

ICSH - acronym for Interstitial Cell Stimulating Hormone an anterior pituitary hormone. (More? Interstitial Cell Stimulating Hormone)

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