Beginnings, Growth and Development|
Colour Coding
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Gastrointestinal Tract
The gastrointestinal tract (GIT) develops primarily from the endoderm, with inputs from the mesoderm (splanchnic mesoderm) and the ectoderm (neural crest cells form its primary nerve plexus). After the endoderm is folded into a blind-ended tube in week 4, it begins to develop into the primary lining of the GIT, while the mesoderm provides its muscular wall and connective tissues components. The GIT is typically partitioned into 3 parts, based on their separate blood supplies: foregut, midgut and hindgut, with the allantois coming off the hindgut. A bud coming off the tract forms the liver; eventually buds appear that form gallbladder and pancreas. The rostral end of the tube ends at the buccopharyngeal membrane, where it interacts with ectoderm directly (no mesoderm in between). This area forms the future mouth. A similar endodermal-ectodermal interaction occurs at the tail – the cloacal membrane. The section of the GIT here is called the cloaca.
Kidney
The kidneys develop from intermediate mesoderm, which lies between the lateral plate mesoderm and the somites. The kidney develops in multiple stages, which occur in a rostrocaudal sequence. The earliest structure to form is the pronephros, in week 4, featuring a pronephric duct with associated nephrogenic mesenchyme. This degenerates early on, leaving only the duct system running down to the cloaca – this becomes known as the mesonephric duct, in the embryo. The next stage is the formation of the mesonephros, also in week 4. Its differentiation is induced by the pronephros. However, the mesonephros is also a transient structure. It provides a template for the adult metanephros, beginning on day 35-37.
Links: 3D Model Movie - Gastrointestinal Tract, Respiratory, Kidney | 3D Model Movie
The above animation is based upon digitised individual embryo slices and selected individual slices and descriptions are linked below. Observe at this early stage the: relationships between gastrointestinal tract, kidney and vascular systems, relative size and position of structures, cloaca, urogenital ridge and mesonephric duct.
E1-F4: Urogenital ridge with broad attachment to dorsal abdominal wall. Medial thickening of coelomic epithelium of urogenital ridge is site of gonad development (prominent in F2). Mesonephric duct.
F4-G4: Follow the caudal course of the mesonephric ducts.
G2-F2: Follow further the caudal course of the mesonephric ducts up into the sacral region of the embryo. In F2 note colon and mesocolon attaching to dorsal body wall; also allantois, umbilical artery, umbilical vein. (Limb etc. missing on one side - artefact). F1: Umbilical vessels. Allantois joining to urogenital sinus. Mesonephric ducts. note coelom joining of ventral and dorsal body walls with rectum in between, and urogenital sinus in ventral body wall. Urorectal (UR) septum in tissue between hindgut and UG sinus. Site of attachment of umbilical cord (connecting stalk component) to the ventral body wall. Note shape of coelom.
E6: Junction of hindgut and urogenital sinus. UR septum continues laterally and caudally ("upwards") as two crests or flanges. Small coelom. Umbilical artery near junction to iliac artery, aorta. Mesonephric duct.
E5: Umbilical artery arcs to side of the coelom. Mesonephric duct. E4: Increase in caliber of mesonephric duct at site where ureteric bud arises. Definitive kidney to medial side. Lateral extensions of UF sinus.
E3-E2: Mesonephric ducts joining UG sinus. Hindgut. Cloacal membrane.
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This next section will look at the development of the internal genital tract using a series of animations and online resources. We will also look at the development of the kidney, as the mesonephric duct is derived from the developing intermediate kidney. Both the male and female genital tracts are formed from differential remodelling of the mesonephric and paramesonephric ducts respecively under the influence of gonadal supporting cells. Note that all movies can be stopped and controlled using the control strip at the bottom of each movie. To open the movie in a new window use the Embryology Movies link. |
This looped animation shows the 3 stages of kidney development in relation to the development of the hindgut region between Week 4 and 5. The earliest stage of kidney development begins up near the pharyngeal arches as the pronephros (cervical nephrotomes) which quickly degenerate, beside these the mesonephric duct begins to form (purple). The next stage is the extensive mesonephros (red) which extends down towards the developing hindgut region (yellow). Associated with the mesonephros is the mesonephric duct. The final stage in kidney development is formation of the metanephros (pink) which will form the adult kidney. Now observe the changes occuring in the hindgut region (yellow) ending in the embryo at the cloaca. The posterior portion (nearest the mesonephros) remains as the gastrointestinal tract. The anterior portion, which extends into the connecting stalk as the allantois, becomes separated from the hindgut and forms the primordia of the urinary bladder, but remains connected at the common urogenital sinus. |
Link: Kidney and Genital
This looped animation shows the next stage in septation of the urogenital sinus between Week 4 and 6. The initial cloaca is divided by the urorectal septum. The posterior portion forms the rectum and the anterior portion forms the primitive urogenital sinus. The superior part of the primitive urogenital sinus forms the urinary bladder, this and the rectum are the same for both sexes. The inferior part of the primitive urogenital sinus forms different structures in males and females. The constricting part forms the prostatic urethra (male) or the membranous urethra (female), beneath this region the expanded space is the definitive urogenital sinus forms the penile urethra (male) or vagina vestibule (female). Note the animation is simplified, as the cloacal membrane appears to break down before the septum can fuse with it. Several different defects in both sexes can occur if this septation process is not correctly aligned (rectourethral fistula, rectovaginal fistula). |
This looped animation shows the development of the male gonad showing medullary sex cords. The paramesonephric duct (red, left) degenerates under the influence of Mullerian duct inhibitory factor (MDIF) secreted by sertoli cells (differentiated by SRY expression). The mesonephric duct (purple) differentiates under the influence of Testosterone secreted by Leydig cells. Within the testes these mesonephric tubules grow towards the medullary sex cords and will form the rete teste. The mesonephric duct extending out of the gonad forms the ductus deferens. The medullary sex cords (orange) form testis cords that later differentiate into solid seminiferous tubules which become hollow and actively produce spermatazoa during puberty. The tunica albuginea (white) covers the testis and bands extend inward to form connective tissue septa. |
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This looped animation shows the development of the female gonad showing cortical sex cords. The mesonephric duct (purple) degenerates, small remnants may remain as epoophoron and paroophoron (in the mesentry of the ovary) and Gartner's cycts (near vagina). The paramesonephric duct (red, left) grows forming the oviduct (uterine horn) and the end opens into the peritoneal cavity and terminates in fimbria (finger-like extensions). Away from the ovary, the two paramesonephric ducts fuse in the midline to form the uterus. The cortical sex cords (orange) form after the primary sex cords degenerate and mesothelium forms secondary cords. The surrounding connective tissue (pink) differentiates to form follicle cells. |
Now directly compare the development of the male and female gonad (these may not be synchronised).
The links below are to animations showing development of the kidney and internal genitalia, they will open in a new window with no text.
Cervical Nephrotomes, Mesonephros and Metanephros (553Kb)
Development of the Renal Collecting System (298Kb)
Development of the Primitive Urogenital Sinus (476Kb)
Trigone (187Kb)
Male Gonadal Development (434 Kb)
Female Gonadal Development (315Kb)
Formation of Uterus and Vagina (706Kb)
Descent of the Testes (221Kb)
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Link to next page in this Practical - Late Embryo (Week 8 stage 22, male) Looks towards the end of embryonic development at an example of male appearance in cross-sections. |
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.
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 |
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.
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)
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)
axial mesoderm - Alternative name for the notochord, an early embryonic structure lying in the midline of mesoderm within the early trilaminar embryo. (More? notochord | Neural Notes | Week 3 Notes)
axial process - The precursor to the notochord. In the early embryo (week 3) epiblast structure extending from the primitive node (Hensens node) crainally, which will eventuall differentiate to for the notochord, the mesoderm structure that later replaces the axial process. (More? notochord | Neural Notes | Week 3 Notes)
buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity). (More? GIT Notes)
buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomadeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane. (More? GIT Notes | Face Notes | Head 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)
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)
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)
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)
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)
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)
gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers have just formed. (More? Gastrulation)
gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes. (More? Gastrulation)
gene - A DNA sequence that is transcribed as a single unit and encodes a single polypeptide (protein) or a set of closely related polypeptides. There are approximately 20,000-25,000 protein encoding genes in the human genome. In each cell, DNA is found within the nucleus and also within mitochondria. (More? DNA 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)
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)
intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom. (More? Coelom Notes | Week 3 Notes)
mesenchyme - Term used to describe the cellular organisation of undifferentiated embryonic connective tissue . Mesenchymal tissue is mainly derived from mesoderm and neural crest, which will form most of the adult connective tissues. This connective tissue organization contrasts with the other main form of cellular organization, epithelial tissue. (More? Musculoskeletal Notes)
mesoderm - The middle layer of the 3 germ cell layers of the embryo. Mesoderm outside the embryo and covering the amnion, yolk and chorion sacs is extraembryonic mesoderm. (More? Musculoskeletal Notes | Week 3 Notes)
neuralation - The general term used to describe the formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate and all the processes that occur therafter. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. (More? Neural Notes | Neural Crest Notes)
neural folds - The central region of the trilaminar embryo ectoderm called the neural plate region folds dorsally, generating two neural folds, which later fuse to form the neural tube. The mid-line depression between the two folds is described as the neural groove. In humans at approximately day 18-19 post-fertilization to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate ->(day 18-19) neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neural groove - The second stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally, beginning in humans at approximately day 18-19 post-fertilization to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate ->(day 18-19) neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neural plate - The first stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neural tube - The third stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)
neurogenesis - The process of formation of the neural system. This begins with segregation of the neural plate from the ectoderm of the trilaminar embryo by folding to form initially the neural groove, which then fuses to form the neural tube (the central nervous system progenitor, brain and spinal cord) and associated neural crest. (More? Neural Note)
notochord - (axial mesoderm) The rod of cells lying in the midline of the trilaminar embryo mesoderm layer ventral to the neural tube. Thought to have at least 2 early roles in development and later roles in patterning surrounding tissues. 1. Mechanical, influencing the folding of the early embryo; 2. Morphogenic, secreting sonic hedgehog a protein which regulates the development of surrounding tissues (neural plate, somites, endoderm and other organs). In humans, the notochord forms in week 3 and is eventually lost during the formation of the vertebral column. (More? Mesoderm Notes | Neural Notes | Molecular Development- Sonic Hedgehog)
paraxial mesoderm - The two lateral strips of mesoderm lying beside the axial mesoderm (notochord). This mesoderm at the body level will segment into somites, at the head level it remains unsegmented. (More? Musculoskeletal Notes | Week 3 Notes)
somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/90 minutes until on average 44 pairs eventually form. (More? Week 3 - Somitogenesis | Musculoskeletal Notes)
spermatazoa - The male haploid reproductive cell, produced by meiosis in the testis (male gonad). (More? Week 1 - Spermatogenesis)
spermatogenesis - (Greek, genesis = origin, creation, generation) The term used to describe the process of diploid spermatagonia division and differentiation to form haploid spermatazoa within the testis (male gonad). The process includes the following cellular changes: meiosis, reoorganization of DNA, reduction in DNA content, reorganization of cellular organelles, morphological changes (cell shape). The final process of change in cell shape is also called spermiogenesis. (More? Week 1 - Spermatogenesis)
splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall. (More? Week 3 | Gastrointestinal Tract Notes | Heart Notes )
sry - (Sry, human; Testis-Determining Factor, TDF; Testis-Determining Factor on Y, TDY ) Gene name sex-determining region of Y, the gene locus on the Y chromosome encoding the male "testis determining factor", a protein transcription factor and a member of the high mobility group (HMG)-box family of DNA binding proteins. See also the transcription factor SRY-related protein, SOX9 (SRY-related high-mobility group (HMG) box 9) (More? Molecular Notes | Week 1 Notes | OMIM)
testes - (Latin testis = "witness") The two male gonads (singular testis) where male germ cells (spermatozoa) are generated and also the source of testosterone (male hormone). Embryonically formed from primordial germ cells entering region of the paired mesonephric ducts (Wolffian ducts) which are preserved in male gonad development and lost in females. (More? Genital System - Male)
testis - (Latin testis = "witness", plural testes) The male gonad where male germ cells (spermatozoa) are generated and also the source of testosterone (male hormone). Embryonically formed from primordial germ cells entering region of the paired mesonephric ducts (Wolffian ducts) which are preserved in male gonad development and lost in females. (More? Genital System - Male)
testis-determining factor - (TDF, Sry, Testis-Determining Factor on Y, TDY ) Protein name for the protein transcription factor product of the Sry gene on the Y chromosome responsible for maleness. This protein is a member of the high mobility group (HMG)-box family of DNA binding proteins. See also the transcription factor SRY-related protein, SOX9 (SRY-related high-mobility group (HMG) box 9) (More? Molecular Notes | Week 1 Notes | OMIM)
X chromosome - The female sex chromosome, which following sexual reproduction is inherited from each parent in females, and inherited from the mother in males. This inheritence pattern impacts upon the pattern of genetic disease. (More? X Chromosome)
Xist - The name for a non-translated RNA (18 Kb) that is associated with the inactivated X chromosome in female cells to correct for the double gene dosage, 2 copies of the X chromosome. (More? Molecular Development - X Inactivation | X Chromosome | Week 1 Notes)
X inactivation - Process that occurs in all cells within females, each cell has 2 copies of the X chromosome (one from father and one from mother) one of copy of which is randomly inactivated throughout the entire body in order to maintain gene dosage. (More? Molecular Development - X Inactivation | X Chromosome | Week 1 Notes)
X linked - Term used to refer to genes, and genetic diseases, located on the X chromosome. Therefore more likely to be expressed in males, where there is only a single maternal X chromosome. (More? X Chromosome)
Y chromosome - The male sex chromosome which contains the sry gene producing Testis-Determining Factor required for male phenotype and can only be inherited from father. In humans the chromosome contains 200+ genes and consists of 50 million base pairs. Testis-Determining Factor (TDF; Testis-Determining Factor on Y, TDY ) is a protein transcription factor and a member of the high mobility group (HMG)-box family of DNA binding proteins. See also the transcription factor SRY-related protein, SOX9 (SRY-related high-mobility group (HMG) box 9). (More? Y Chromosome | Molecular Notes | Week 1 Notes | OMIM)
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This is a new BGD Practical under development. (This notice removed when complete)
This class is going to take you through the earliest steps in Sexual Differentiation using linked online resources you will use in the actual class.
The lefthand side menu has links to all pages (in sequence) required for this Lab, alternatively follow the white rabbit.... Also at the bottom of the same menu is a window to Search UNSW Embryology using any term/concept you do not understand. (note the search window uses Google, so will only work with an active internet connection.
If you are interested in a more detailed coverage, look at the Early Development 1 and Early Development 2 sections of Notes (from the lefthand menu).
These notes and linked materials have been prepared for Educational purposes only.Please email Dr Mark Hill if you wish to make a comment about this current project.