Embryology Flashcards
How many oocytes does a female 20 week foetus have
6-7 million
How many oocytes does a female have at birth?
1-2 million
How many oocytes does a female have at puberty?
300,000
When after fertilisation does the morula appear?
Day 4
When after fertilisation does the blastocyst appear?
Day 5
From what do the primordial germ cells arise?
Wall of the yolk sac in the 2nd week of development
What do primordial germ cells form in females?
Oogonia
What do primordial germ cells form in males?
Spermatogonia
When do the primordial germ cells migrate to occupy the gonadal ridges?
Week 6 of development
What happens to oogonia in the fetal period?
Oogonia proliferate by mitosis and differentiate into primary oocytes, however, these oocytes are then suspended in prophase I (I, i.e. the first meiotic division) from birth until puberty
At what stage of meiosis are oogonia suspended in from birth until puberty?
Prophase I
What causes the arrest in meiosis experienced by the primary oocytes during the foetal period?
Oocyte maturation inhibitor (OMI) - a peptide produced by the follicular cells surrounding a primary oocyte
What are the layers of the follicle that surround the primary oocyte?
1) Theca interna - the inner vascular and secretory layer
2) Theca externa - the outer fibrous layer
Where do the spermatogonia lie dormant in males, and until when do they remain dormant?
Spermatogonia remain dormant in the seminiferous tubules in the testes until puberty
How soon after ovulation does fertilisation take place?
12-24 hours after ovulation
What the summarised stages of fertilisation?
1) Sperm activation and penetration of the corona radiata
2) Attachment and penetration of the zona pellucida
3) Fusion of the oocyte and sperm cell membranes
4) Completion of meiosis of the oocyte and formation of the pronuclei
5) Formation of the zygote
What process do sperm need to undergo to be capable of penetrating the ovum?
Capacitation
What happens during capacitation?
Loss of glycoprotein and cholesterol from the acrosomal membrane by secretions from the cervix and uterine tube
How do sperm penetrate the corona radiata?
By the acrosomal reaction, releasing hyaluronidase
How do sperm penetrate the zona pellucida?
Also by release of the acrosomal enzymes - esterases, acrosin and neuraminidase
What is the zona reaction?
When the sperm binds with the zona pellucida
What is the point of the zona reaction?
It changes the physical properties of the zona pellucida to prevent attachment/penetration of any further sperm
What causes the zona reaction?
The release of lysosomal enzymes from the cortical granules into the privitelline space
When does the oocyte resume its second meiotic division?
Soon after entry of the spermatozoa
How do you know that the second meiotic division has occurred?
The 2nd polar body is visible down the microscope
What is the zygote?
The zygote is the product of complete fertilisation/ It occurs when the male and female pronuclei have met, their membranes broken down and their chromosomes mixed to create a single cell - this single cell is the zygote
What are the components of the blastocyst at day 5?
Trophoblast = outer cell layer, from which foetal component of placenta will form Embryoblast = inner cell mass, from which embryo will form Blastocele = cavity inside the blastocyst
How many days after fertilisation does implantation occur?
Approx. 6
When does the trophoblast differentiate, and what does it differentiate into?
Week 2
Cytotrophoblast = inner, single layer of cells
Syncytiotrophoblast = outer layer. = more invasive but has not yet invaded the endometrial blood vessels
Decidualisation if the uterine stroma forms the maternal component of the placenta (separate reaction)
When does the embryoblast/inner cells mass of the blastocyst differentiate, and what into?
Also week 2
2 layers - the epiblast and the hypoblast. These form the bilaminar disc
Where does the amniotic cavity form?
In the epiblast layer, developing at approx. 8 weeks. The amniotic cavity is filled with amniotic fluid.
What does the hypoblast (of the bilaminar disc) develop into?
The exocoelomic membrane is derived from the hypoblast and forms the primary yolk sac.
What is the state of the trophoblast by day 12?
The syncytiotrophoblast have formed lacunae which are responsible for derive ng nutritional support from the endometrium.
What is the state of the trophoblast by day 13?
The cytotrophoblast forms primary chronionic villi - finger like protrusions into the lacunae of the syncytiotrophoblast.
When does the primary yolk sac become the secondary yolk sac?
Day 13
What is the extra-embryonic mesoderm?
A connecting stalk between the embryo and cytotrophoblast. It is the forerunner of the umbilical cord.
What is the state of the trophoblast by day 14?
The syncytiotrophoblast begins to produce Beta-hCG to maintain the corpus luteum so that the corpus luteum can maintain the thickness of the endometrium.
What is the name of the process by which three germ layers are formed?
Gastrulation
What are the names of the three germ layers?
Epiblast —> 1) Ectoderm
From the ectoderm you also get the 2) Mesoderm
Hypoblast —> 3) Endoderm
When do the three germ layers develop?
3rd week
What forms from the ectoderm?
1) Surface ectoderm - epidermis, tooth enamel, lens, internal ear
2) Neural tube - CNS; retina; posterior pituitary
3) Neural crest - facial bones, adrenal medulla, cranial and sensory ganglia
What forms from the mesoderm?
1) Paraxial mesoderm - skeletal muscles of trunk and limbs, skeleton (except skull), dermis, connective tissue
2) Intermediate mesoderm - urogenital system
3) Lateral plate mesoderm - smooth muscles and connective tissue, viscera
What forms from the endoderm?
Gut tube and liver
After the development of the 3rd germ layer - i.e. the mesoderm - at what points do the ectoderm and endoderm remain in contact?
1) Prochordal plate - at the cephalic end of the disc —> becoming the buccopharyngeal membrane
2) Cloachal plate - at the caudal end of the disc —> becoming the cloachal membrane
What is the primitive streak?
A groove like depression in the bilaminar disk that forms at the caudal end in week 2, before it begins the deepen and extend in week 3 at the three germ layers develop
Where is the primitive node?
At the cephalic end of the primitive streak
What develops from the cells of the primitive node?
The notochordal plate, which then folds to form the notochord, which then underlies the future neural tube to ultimately form the nuceli pulposi (centres) of the intervertebral discs of the vertebral column
What is the name of the process that forms the brain and spinal cord?
Neurulation
What are the stages of neurulation?
Day 19 - neural plate formed from ectoderm, rounded up to form neural folds
Day 20 - neural plate deepens to form the neural groove, from which you get the neural tube
Where do neural crests form?
At the edge of the neural tube where the neuro-ectoderm is continues with the surface ectoderm
What is the connection between the yolk sac and the gut?
Vitello-intestinal (vitelline) duct.
Which develops first, the urinary system or the genital system?
Urinary BEFORE genital
Where do the urogenital ridges develop?
In the mesoderm, either side of the aorta
What are the different parts of the urogenital ridges?
Nephrogenic cord - develops into urinary system
Gonadal ridge - develops into genital system
What forms from endoderm?
Stomach Intestines (mucosal layers) Colon (mucosal layers) Liver Pancreas Bladder Epithelial parts of trachea Lungs Pharynx Thyroid Parathyroid
What forms from intermediate meosderm?
Kidneys
Gonads
What forms from paraxial mesoderm?
Cartilage
Skeletal muscle
Dermis
What forms from lateral plate mesoderm?
Circulatory system (inc heart) and
Spleen
Gut wall
What forms from neural crest of ectoderm?
Peripheral nervous system Adrenal medulla Melanocytes Facial cartilage Dentin of teeth
What forms from neural tube of ectoderm?
Brain Spinal cord Posterior pituitary Motor neurons Retina. Anterior pituitary
What is the incidence of Mullerian Agenesis AKA Rokitansky-Kuster-Hauser syndrome?
1/5000
What are the 2 conditions that cause a congenital absence of the uterus?
1) Mullerian Agenesis AKA Rokitansky-Kuster-Hauser syndrome
2) CAIS AKA Testicular Feminisation Syndrome
How do you differentiate between Mullerian Agenesis AKA Rokitansky-Kuster-Hauser syndrome and CAIS AKA Testicular Feminisation Syndrome?
Mullerian Agenesis AKA Rokitansky-Kuster-Hauser syndrome = normal hormone levels
CAIS AKA Testicular Feminisation Syndrome = low E2
What is ootidogenesis?
The 2 meiotic divisions the primary oocytes undergo to become ootids
What is oocytogenesis?
Growth and maturation of oogonium to primary oocytes
What are Ladd’s bands?
When intestinal malrotation causes fibrous bands that fix the caecum in the right upper quadrant
How is the foregut innervated?
T5-T9
How is the midgut innervated?
T10-T11
How is the hindgut innervated?
T12-L1
What is the most important chemical for maintaining a patent DA during pregnancy?
PGE2
Where is PGE2 produced in pregnancy?
Placenta
When does meconium first appear in the terminal ileum?
10-16 weeks gestation
What does the corpus luteum produce?
1) Progesterone
2) Estradiol
3) Inhibin A
What is the maturation process for sperm?
Spermatocytogenesis - spermatagonium to primary spermatocytes
What is spermatidogenesis?
When spermatocytes undergo 2 meiotic divisions to become spermatids
What is spermiogenesis?
When spermatids differentiate into spermatazoa
What is the life span of the corpus luteum?
14-16 days
What is a Gartner’s duct?
A cystic structure found lateral to the vagina
It’s embryological origin is the mesonephric duct
From what does the intraembryonic coelom form?
Lateral plate mesoderm
What is the canal of Nuck associated with?
Round ligament
At what stage does implantation occur?
7-9 days, i.e. when the embryo is a blastocyst
What condition occurs is the yolk sac persists?
Meckel’s diverticulum
From what structure do the kidneys and renal tract develop?
Metanephros
From what germ layer do the autonomic ganglia develop from?
Neural crest
A bicornate uterus results from a failure of fusion of what?
Paramesonephric ducts
From what germ layer are the ureters derived?
Mesoderm
From where do primordial germ cells originate?
Yolk sac
From which embryological structure does the penis develop?
Genital tubercle
What develops first, the urinary system or genital system?
Urinary system
What are the components of the urinary system?
Pronephros - week 4 appearance, never functional
Mesonephros - week 4 appearance, functions between weeks 6-10
Metanephros - week 4 appearance, functions from week 12 as the permanent kidney
What are the 2 parts of the metanephros?
- Ureteric bud
- Metanephric mesoderm
What is derived from the ureteric bud?
- Collecting duct
- Minor and major calyx
- Renal pelvis
- Ureter
What is derived from the metanephric mesoderm?
- Renal glomerulus
- Renal capsule
- PCT
- Loop of Henle
- DCT
From what embryological structure are the bladder, urethra, urethral/paraurethral glands and vagina derived?
Urogenital sinus
From what embryological structure are the labia minora and ventral aspect of the penis derived?
Urethral folds
From what embryological structure are the labia majora and scrotum derived?
Genital swellings
From what embryological structure are the ovarian and round ligament derived?
Gubernaculum
As the kidney ascends from the pelvis, by how many degrees does it rotate?
90 degrees
In whom, and on which side is renal agenesis more common?
In males, and on the L side
What is renal agenesis caused by?
When the ureteric bud fails to induce the metanephric mesoderm
When does duplication of the ureter occur?
It occurs as a result of early splitting from the ureteric bud
Where is the SRY gene located?
The short arm of chromosome Y - Yp11
From what does a Gartner’s cyst derived (found in the uterus/vagina)?
When the caudal part of the mesonephric ducts persists
What is the time frame for the journey the testis make into the scrotum?
Week 12 - testis reach the inguinal canal
Week 28 - testis migrate through the inguinal canal
Week 33 - testis reach the scrotum
What are the fascial layers of the testis, and from what are they derived?
Internal spermatic fascia - from transversalis fascia
Cremesteric fascia - from internal oblique
External spermatic fascia - from external oblique
When does the primitive gut form?
Week 4
What structures arise from the foregut?
Oesophagus Trachea and lung buds Stomach Proximal duodenum Liver, biliary tree, pancreas
What structures arise from the midgut?
Distal duodenum
Small intestine
Caecum and appendix
Ascending colon and two thirds of the transverse colon
What structures arise from the hindgut?
Distal third of the transverse, descending and sigmoid colon
Rectum and superior part of anal canal
Epithelium of urinary bladder and most of urethra
What does the pectinate line (also called the white line) denote?
The junction between the endoderm of the hindgut and the ectoderm of the proctodeum, dividing the upper two thirds and the lower third of the anal canal
What does the umbilical vein form after birth?
Ligamentum teres hepatis
What does the ductus venosus form after birth?
Ligamentum venosum
What develops from the vitelline vein?
Hepatic veins
Inferior part of IVC
Superior mesenteric vein
Portal vein
From what does the blastocyst hatch?
Zona pellucida
At what gestation does a conceptus become a fetus?
9 weeks
How many cells are there in a blastocyst?
70-100 cells
When does the buccopharyngeal membrane breakdown?
Week 4
When does the cloacal membrane breakdown?
Week 7
From when is the first heartbeat noted?
Day 21
When does embryo folding occur?
Days 21-24
From when are spontaneous limb movements first noticeable on USS?
Week 7
What is the maximum number of somites, and what is their rate of production?
Max 42-44 pairs
Rate of 3 pairs/day
What are the layers of the decidua?
Basalis
Capsularis
Parietalis
What is the name for the changes in the endometrial glands as a result of progesterone?
Arias-Stella
What are the 2 units of the placenta?
- Chorionic plate - is fetal in origin - consists of a) amnion; b) extra-embryonic mesenchyme; c) cytotrophoblast; d) syncytiotrophoblast
- Basal plate - has a fetal derived part (from the cytotrophoblast and syncytiotrophoblast) and a maternal derived part (from the decidua)
What divides the placenta into lobules?
Placenta septa (inter-cotyledon septa), which arise from the decidua at 4 months gestation
What are the possible genotypes of partial molar pregnancies?
In order of decreasing frequency:
69XXY
69XXX
69XYY
From what are the body cavities - i.e. the peritoneal/pleural/pericardial - derived?
Coelomic cavity
What are the body cavities lined by?
Parietal layer of serous membrane - derived from somatic mesoderm
Visceral layer of serous membrane - derived from splanchnic mesoderm
What are the types of ossification?
Membranous ossification - development of flat bones
Endochondral ossification - developmental of long bones
When does ossification start and end?
Weeks 6-25
The inner nucleus pulposus is derived from notochord, what is the outer nucleus pulposis derived from?
Sclerotomes
What are fontanelles?
Unossified mesenchyme
What does each myotome divide into?
Ventral hypomere - muscles of anterior wall
Dorsal epimere - muscles of posterior wall
What muscle group does dorsal limb muscle mass give rise to?
Extensor group
What muscle group does ventral limb muscle mass give rise to?
Flexor group
What suspends the gut tube in the body of embryo?
Dorsal mesentery
When does gut herniation occur?
Herniates at week 6, and then returns at week 10
What is the rotation of the gut from herniation?
90 degrees anticlockwise as it herniates into the umbilical cord
Further 180 degrees anticlockwise when it returns to the abdomen
Which is associated with chromosomal abnormalities - omphalocele or gastroschisis?
Omphalocele
What does Meckel’s diverticulum contain?
Gastric mucosa
Pancreatic mucosa
What does the allantois become?
The urachus (which itself becomes the median umbilical ligament)
What is the bladder trigone formed from?
Mesonephric duct
From what is the bladder formed?
Urogential sinus
From what are Leydig cells derived?
Gonad mesenchymal cells
From what are granulosa cells derived?
Primitive sex cords
What is the severity of the ambiguity of gonads classified by?
Prader stages 1-5
