Case 14- Embryology 2 Flashcards
Neurulation (days 17/18-19)
The neural plate and ectoderm move towards the midline, the lateral edges of the neural plate rise up making the neural folds. Between the folds is the neural groove. Further folding of the neural plate produces a deeper neural groove. Neurulation and mesoderm induction happens at the same time. The Mesoderm differentiates into 3 types
Types of mesoderm induction
• Paraxial mesoderm - differentiates into sclerotome and dermomyotome, both become muscle
• Intermediate mesoderm
• Lateral plate mesoderm - Intraembryonic coelom forms within the splanchic and somatic
The paraxial mesoderm is nearest the Notochord and receives the most signal
Neurulation (days 20-21)
The neural tube is completely rolled up and internalised. The surface ectoderm remains external and covers the neural tube. The neural crest constitutes the remaining neural plate cells. The paraxial mesoderm now differentiates into three parts. The lateral plate mesoderm differentiates into two parts. The notochord regresses, becomes part of the intervertebral disc in adults.
Differentiation of the lateral plate mesoderm
Splanchic and somatic mesoderm
Differentiation of the Paraxial mesoderm
Sclerotome, Dermomytome and Myotome
Intramebryonic coelom
The gap between the Splanchic and Somatic mesoderm, form the body cavities (pericardium, pleura and peritoneum)
What is formed from the lateral plate mesoderm
Splanchic mesoderm- heart, mandibular muscles
Splanchopleure- gut walls
Somatic mesoderm- limb buds
Somatopleure- body wall
Location of notochord and neural plate
The notochord is the midpoint of the mesoderm. The area of the Ectoderm above the notochord differentiates into the neural plate. The further you get from the notochord the less strong the signals are
Neurulation
Occurs between 15-28 days of development, the lateral edges of the neural plate elevate resulting in the formation of neural folds. The neural folds meet and fuse to form the neural tube, which forms the spinal cord and the brain. The neural crest cells migrate away to become PNS, Schwaan cells, melanocytes, endocrine cells, craniofacial structures and cardiovascular structures. Genetic defects in neural crest cells can cause craniofacial and cardiac defects.
Folding (embryology)
The 2D disk forms the 3D structure. The neural tube proliferates at a faster rate then the rest of the disk causing folding. Folding occurs between days 22-28.
Oropharyngeal cavity
Forms the mouth
Cloaca
Forms the anal cavity
Connecting stalk
At the cranial end, forms the umbilical tube
Process of folding
The ectoderm grows faster then the Mesoderm and Endoderm causing longitudinal folding. Causes the heart to move from the cranial end of the embryo to the thorax. Longitudinal folding is folding at the cranial and caudal end. Longitudinal folding causes internalisation of the gut tube.
Transverse folding
Where the ectoderm and the mesoderm fold down to the endoderm. Results in the endoderm being compressed and internalised. The midgut becomes internalised and is associated with the peritoneal cavity.
How do body cavities form
From from the intraembryonic coelom with lateral plate mesoderm. The anterior portion becomes the pericardial cavity (heart) and the pleural cavity (lung). The posterior portion becomes peritoneal cavity. Explains why they all have similar structures, double membrane with fluid in cavity.
Types of body cavities
Lung pleura, pericardium, serous pericardium around the heart and the serous peritoneum around the gut
When do body cavities form
Days 22-28 when the body is folding
How does the lateral plate mesoderm split
Into the Splanchic and Somatic mesoderm, between them is the intraembryonic coelom. The lateral plate mesoderm forms a horseshoe shape with none at the caudal end
Chorion
First form of the placenta. The chorion is derived from trophoblasts and splits into the smooth chorion (chorion leave) and villus chorion (chorion frondosum)
Time period of placental villus formation
Days 5-22. At the same time as implantation, gastrulation and Neurulation. Occurs after implantation as the chorion is derived from trophoblasts.
Extraembryonic tissue (foetal membrane)
Yolk sac and allantois, amnion, chorion
Placental villus formation (days 12-14)
The chorion is derived from both the Cytotrophoblast and Syncytiotrophoblast. In the Syncytiotrophoblast the lacunae link up with the maternal blood supply. The embryo is embedded within the uterine wall (the Decidua).
Placental villus formation (Days 16)
The primary villus forms, which is an elongation of the Cytotrophoblast through the Synctiotrophoblast. Due to Cytotrophoblast proliferation and migration through the space the Synctiotrophoblast used to be in.
Placental villus formation (day 17)
The secondary villus forms. The extraembryonic mesoderm contributes to the chorion and the villus. The extraembryonic mesoderm proliferates and invades the space that was the primary villus. Some of the primary villus remains.
Placental villus formation (weeks 8-15)
The villi (villus chorion) are located only on one side. On the other side is just the smooth chorion. The smooth chorion fuses with the amnion which is surrounding the developing foetus. There is a rich blood supply through the villus chorion but there is a poor blood supply through the smooth chorion so the villi degenerate.
Amniochorion
The fusion of the amnion and the smooth chorion. It causes the fusion of the decidua paretallis and capsularis, results in loss of uterine lumen due to fusion of decidua and growth of foetus. The placenta is formed from the proliferation of the villus chorion and has an excellent blood supply.
Acessory placenta
Blood supply remains in the placenta. Its when the villi and placenta do not degenerate. Can cause post partum haemorrhage if the placenta is covering the placenta
Chorionic plate
Separates the mother from the foetus, the umbilical vessels run through it. You have the interface between the maternal and foetal circulation through the lacunae. The umbilical vein brings in oxygen to the foetus and the umbilical artery takes waste products away.
Placenta structure
Villus arteries and veins form in the extraembryonic mesoderm. The villi are bathed in the maternal blood. The lacunae are lined by the Syncytiotrophoblast. There is physiological exchange of substances between the lacunae and the villi. The maternal blood enters the lacunae via uterine spinal arteries. The maternal blood filters down past the villous chorion. Within the umbilical cord and each villus there are two umbilical arteries and one umbilical vein.
What happens to the placenta nearer delivery
More of the Cytotrophoblasts have differentiated into Synctiotrophoblasts
Placenta function
- Respiratory: O2 from mother - foetus
- Waste: CO2, urea, billirubin (Hb breakdown)
- Nutrition: Glucose, amino acids, free fatty acids
- Hormone production
- Immune barrier
- Antibodies
- Drugs
What forms the tertiary chorionic villus
The extra-embryonic mesoderm migrates in to form it.
How is genetic sex determined
(XX, XY) at fertilisation. By absence or presence of the SRY gene. Also regulated by hormones and glycoprotein factors.
Reproductive system development- Indifferent stage
When the reproductive system could differentiate into either males or females
What does the reproductive system develop from
Intermediate mesoderm, along with the urinary system
What does SRY result in the formation of (brief)
Spermatozoa, the male genital tract and external male genitalia
How is male genitalia formed
- The Y chromosome codes for the SRY gene
- This turns the indifferent gonads into testes
- Differentiation within the testes produces Leydig cells, Sertoli cells and Spermatogonia
- The Leydig cells produce testosterone which causes the Mesonephric duct to produce the Male genital tract (Ductus deferens, Epididymis and the seminal vesicle)
- Testosterone is converted to Dihydrotestosterone (DHT) by 5alpha-reductase. DHT produces the external male genitalia.
- It causes the Urethral epithelium to form the prostate, the Genital swellings to become the Scrotum and the Genital tubercle to form the glans Penis. The genital folds become the penis
What do the Sertoli cells do (male development)
The Sertoli cells produce the androgen binding factor, which converts Spermatogonia into differentiated Spermatozoa with testosterone. The Sertoli cells also produce the Mullerian inhibiting substance, which degenerates the Paramesonephric duct.
How is the female reproductive tract formed
In the absence of Mullerian inhibiting substance, the paramesonephric duct persists and differentiates into the uterine tube, uterus and vagina. The urogenital sinus differentiates into the paraurethral and greater vestibular glands, and the lower genital tract. The mesonephric duct degenerates to form Gartner’s duct. The Mesonephric duct is formed in kidney development. Female genital tract is formed after 8 weeks. It is the default system
How are the germ cells formed
Arise during week 3-4 in the Allantois within the yolk sac. The Allantois is in the hindgut
Primordial germ cells
Precursors of germ cells, become oogonia or spermatogonia in adults. Primordial germ cells originate in the urogenital ridge and then travel to the gonads via the dorsal mesentery. The gonadal ridge is the site of development of the gonads. The Gonadal ridge is made of Germinal epithelium and Mesenchyme (mesoderm), these cell types differentiate into the gonads.
Indifferent stage of gonadal differentiation
1) Germ cells have migrated
2) Germinal epithelium has developed into cords
3) SRY (sex determined region of the Y chromosome) is responsible for further development of the gonads
Male Gonad formation
In males the cortex degenerates. The medulla remains and proliferates, forming the testes. The germ cells become spermatogonia and the tubules and channels form within the testes. The tunica albuginea forms the covering around the scrotum. The Tunica albuginea is the first sign of maleness
Female gonad formation
In the absence of SRY the ovaries form. The cortex remains and proliferates and the medulla degenerates at 10 weeks. After 14 weeks the cortical cords degenerate. The Oogonium is surrounded by follicular cells. There is formation of a reserve of primordial follicles. In the medulla the tubules degenerate forming the Rete ovarii.
At what stage is the external genitalia indifferent
Week 7, the male external genitalia forms at week 9. The female external genitalia forms at week 11
How does the male external genitalia form
Dihydrotestosterone causes the growth and fusion of genital folds. This encloses the urethra to create the penis.
1) The Glans penis forms from the genital tubercle.
2) The Scrotum forms from the genital swelling.
3) The Scrotal raphe forms from the Line of fusion.
4) The external genitalia forms at week 9.
How does the female external genitalia form
No DHT. There is no fusion of the urogenital folds,
1) The labia minora forms from the genital folds.
2) The Glans clitoris forms from the Genital tubercle.
3) The Labia majora forms from the genital swelling.
4) The external genitalia forms from week 11.
What does the urogenital sinus differentiate into
The Prostate, urethral and bulbourethral gland
Consequence of the Absence of an X chromosome in females
- Turner’s syndrome
- Primordial germ cells degenerate
- Gonads do not differentiate
- Genitalia do not mature
An extra X chromosome in males
- XXY Klinefelter syndrome
- Small external genitalia
- Infertility (azoospermia/oligospermia)
Genetically intersex
Extremely rare. 46 XX/XY with an SRY mutation or other anomalies. The external genitalia appears female. The individual possesses both male and female tissue with gonads ‘ovotestis.’
Hormonally intersex
Abnormal sex hormone levels during development. You have genetic females (46, XX) with male external genitalia and genetic males (46, XY) with underdeveloped external genitalia).
Hormonally intersex- absence of SRY
The absence or mutation of SRY can lead to abnormalities in testis development, and therefore would impact upon the development of Leydig cells, and therefore the production of testosterone and dihydrotestosterone (DHT). In the absence of DHT, the indifferent genitalia differentiate into female external genitalia.
De la Chapelle syndrome
Causes the development of male external genitalia in a genetic female. Due to the presence of SRY within an X chromosome
Testicular feminisations
Androgen insensitivity syndrome. Failure in testosterone receptors, they are genetically male (46, XY) but the testicular tissue presents internally. You have an external female phenotype even though testosterone and Mullerian inhibitory substances are produced. Both the Paramesonephric duct degenerates and the Mesonephric duct. Results in infertility.
What surrounds the germ cells in the primordial ridge
Mesenchymal stroma
What surrounds the oocyte in the oocyte in the primary follicle
Granulosa
What do thecal cells release
They are in the maturing follicle and release androgens
What surrounds the mature oocyte
The Cumulus oophorus
Abnormalities of testicular descent
- Undescended testes (cryptorchidism)- the testes do not migrate from the abdomen into the scrotum. It normally migrates through the processus vaginalis
- Ectopic testes
- Infertility due to suboptimal temperatures
Congenital inguinal hernia
The abdominal contents protrude into the patients processus vaginalis, when it does not close fully
Failure of fusion in the genital folds of men
- Hypospadias- where the urethra opens onto the ventral surface of the penis
- Epispadias- urethra opens onto the dorsal surface
Female genital tract abnormalities (abnormal fusion of the Paramesonephric ducts)
- Double uterus
- Separate uterus
- Unicornuate-drains into a single fallopian tube
- Atresia of cervix
- Bicornuate- composed of two horns separated by a septum
