27-28. Anatomy | Early Embryology I-II

What are primordial germ cells?

~40 undifferentiated cells, omnipotent in early embryo. Originate near yolk sac, migrate via umbilical cord into embryo→ gonads (future ovary/testis). Migration guided by chemical signals. Fail to reach gonads→ sterile. PGCs are 'the next generation already inside the grandmother.'

In males, what happens to primordial germ cells after migration?

They enter the testes and remain dormant until puberty, when meiosis begins. Continuous sperm production until old age. Each cycle produces 4 spermatozoa.

In females, what happens to primordial germ cells after migration?

They enter the ovaries and divide by mitosis into millions. Meiosis begins at 5 months in utero but arrests until fertilization. Each cycle produces 1 ovum + 3 polar bodies. ~400 ova total.

What surrounds the ovum during ovulation?

Cumulus oöphorus (follicular cells + zona pellucida).

What membrane ruptures during ovulation?

Visceral peritoneum of the ovary.

How is the ovum transported to the oviduct?

Oviduct fimbriae and cilia generate suction to draw it into the tube.

What happens if the ovum misses the oviduct?

If unfertilized: harmless, lost in abdomen. If fertilized: ectopic pregnancy (life-threatening).

Where does fertilization occur?

In the ampulla of the upper oviduct.

What happens to the ovum at fertilization?

Completes meiosis II, restores diploid 2N, ensures genetic recombination for uniqueness.

What are the early divisions after fertilization?

Zygote→ morula→ blastocyst.

What are the two components of the blastocyst?

Trophoblast (outer cells, nutrition, placenta) and Inner cell mass (embryo + reserved primordial germ cells).

Where does implantation usually occur?

Upper uterus (preferred site).

What happens if implantation fails?

The embryo is expelled with menstruation.

What hormone does the syncytiotrophoblast secrete, and when?

hCG by day 9 after fertilization.

What is the role of hCG?

Maintains corpus luteum graviditatis, which sustains progesterone and preserves the endometrium.

What happens if prostaglandins act at implantation stage?

Corpus luteum regresses→ abortion.

What are the two layers of the bilaminar embryo?

Epiblast (upper, adjacent to amnion) and Hypoblast (lower, adjacent to yolk sac).

What is the chorionic cavity and its subdivisions?

Encloses bilaminar embryo; Smooth chorion on one side, Villous chorion on placenta-forming side.

How does the trilaminar embryo form?

Epiblast cells migrate via primitive streak and node→ displace hypoblast (endoderm), create mesoderm in middle, remaining epiblast becomes ectoderm.

What is a diverticulum in embryology?

A pouch/tube branching from another tube, mechanism of organ formation (e.g., gut→ liver, pancreas, lungs, teeth, skull bones).

What is the notochord and its function?

A rod of cells from ectoderm; defines cranial-caudal and left-right axis, organizes paraxial mesoderm into somites. Adult remnant: nucleus pulposus of intervertebral discs.

What is neurulation?

Neural plate folds to form neural tube (CNS); neural crest cells break off and migrate widely.

What are derivatives of neural crest cells in the nervous system?

Dorsal root ganglia, cranial nerve ganglia, postganglionic neurons, Schwann cells.

What endocrine structure derives from neural crest?

Adrenal medulla.

What skeletal structures derive from neural crest?

Skull flat bones, teeth (dentine, enamel).

What pigment cells derive from neural crest?

Melanocytes.

What other structures derive from neural crest?

Meninges of brain/spinal cord, dermis of face/neck.

What are the subdivisions of mesoderm and their derivatives?

Paraxial mesoderm: somites (sclerotome, myotome, dermatome).

Intermediate mesoderm: embryonic kidneys, reproductive ducts.

Lateral plate mesoderm: splits into splanchnopleure and somatopleure; forms celoms, smooth muscle, periosteum, limb bones.

What are somites?

Repeating blocks of paraxial mesoderm that subdivide into sclerotome, myotome, and dermatome.

What does the sclerotome form?

Vertebrae and intervertebral discs (annulus fibrosus from somite + nucleus pulposus from notochord).

What does the myotome form?

Epaxial muscles (back) and hypaxial muscles (body wall, limbs, ribs).

What does the dermatome form?

Dermis; clinical basis for dermatome maps.

What is re-segmentation of somites?

Caudal half of one somite fuses with cranial half of next, allowing spinal nerves to pass through intervertebral foramina.

What is the origin and function of the amnion?

Origin: epiblast. Function: filled with amniotic fluid (fetal urine + maternal plasma), cushions fetus, necessary for face and limb development.

What is the origin and function of the yolk sac?

Origin: hypoblast. Provides early nutrients, site of first blood cells, connects via vitelline duct & vessels.

What is the origin and function of the chorion?

Origin: trophoblast. Forms placenta (villous chorion); encloses amnion + yolk sac.

What is the origin and function of the placenta?

Origin: maternal endometrium + chorion. Function: exchange of nutrients/waste, produces hCG and progesterone.

How are celoms formed?

By splitting of lateral plate mesoderm into splanchnopleure (visceral) and somatopleure (parietal).

What body cavities are formed from celoms?

Right pleural, Left pleural, Pericardial, and Peritoneal cavities.

How do long bones form?

Periosteal diverticula from somatopleure invade limbs→ ossify→ bones.

How does the heart develop initially?

Starts anterior to brain; folding swings heart under head; begins as beating tube.

How does the GI system form?

Yolk sac diverticulum→ gut tube; branches into stomach, intestines, liver, gallbladder, pancreas, appendix.

What are the five types of nervous system in embryology?

Enteric (oldest), Central (CNS), Peripheral (spinal nerves), Autonomic (sympathetic + parasympathetic), Cranial nerves (12 pairs).

What is an ectopic pregnancy?

Fertilized ovum implants outside uterus. Risk of hemorrhage, often mimics appendicitis.

What causes infertility related to germ cells?

Failure of primordial germ cells to migrate→ gonads sterile.

What hormone is detected in pregnancy tests?

hCG secreted by syncytiotrophoblast, detectable in urine & blood.

What is Meckel's diverticulum?

Remnant of vitelline duct. May cause abdominal pain, mimics appendicitis.

What causes shingles?

Reactivation of varicella-zoster in dorsal root ganglia→ dermatomal rash & pain.

What are amniotic fluid disorders and their causes?

Oligohydramnios (renal agenesis→ Potter sequence); Polyhydramnios (esophageal/duodenal atresia).

What happens during Week 1 of embryonic development?

Fertilization in ampulla, zygote forms, cleavage divisions→ morula→ blastocyst, implantation begins.

What happens during Week 2 of embryonic development?

Bilaminar embryo forms (epiblast + hypoblast), amnion and yolk sac develop, chorionic cavity present, syncytiotrophoblast secretes hCG.

What happens during Week 3 of embryonic development?

Gastrulation→ trilaminar embryo (ectoderm, mesoderm, endoderm). Primitive streak, notochord, and neural plate form. Somites begin to appear.

What key events mark the transition from Week 3 to organogenesis?

Neurulation forms neural tube, neural crest cells migrate, mesoderm differentiates (paraxial, intermediate, lateral plate), heart tube begins beating, GI diverticula form.

By the end of Week 3, what foundational structures are established?

Ectoderm, mesoderm, endoderm layers; notochord; neural tube; somites; early celomic cavities; primitive heart tube.

A newborn is noted to have sterility due to an absence of functional gametes. Further genetic studies reveal that primordial germ cells failed to migrate during embryogenesis. Which of the following is the most likely explanation?

A. The gonads were unable to produce gametes independently
B. Failure of ovulation due to absent FSH signaling
C. Lack of zona pellucida formation around oocytes
D. Arrest of female meiosis at metaphase I
E. Incomplete development of the Müllerian ducts

A

Primordial germ cells (PGCs) must migrate into the developing gonads. If migration fails, the gonads cannot generate gametes on their own, leaving the individual sterile.

• D. Arrest at metaphase I is normal in females until fertilization, not a cause of sterility at birth.

• B, C, E – related to later reproductive physiology or duct development, not PGC migration failure.

A 35-year-old pregnant woman undergoes routine ultrasound. The physician explains that the placenta produces a hormone that maintains progesterone secretion from the corpus luteum in early pregnancy. Which hormone is being described?

A. Follicle-stimulating hormone (FSH)
B. Progesterone
C. Human chorionic gonadotropin (hCG)
D. Luteinizing hormone (LH)
E. Prostaglandin E2

C

Syncytiotrophoblasts secrete hCG beginning ~day 9 post-fertilization. hCG signals the corpus luteum graviditatis to keep secreting progesterone, maintaining the endometrium.

• A. FSH stimulates follicular development, not corpus luteum maintenance.

• B. Progesterone is produced, not the signaling hormone.

• D. LH maintains corpus luteum in non-pregnant cycles, replaced by hCG in pregnancy.

• E. Prostaglandins cause luteolysis → abortion

A 2-year-old boy presents with intermittent abdominal pain and rectal bleeding. Imaging reveals a remnant of the vitelline duct attached to the ileum. Which of the following is the most likely diagnosis?

A. Hirschsprung disease
B. Meckel’s diverticulum
C. Duodenal atresia
D. Omphalocele
E. Malrotation of the gut

B

• Meckel’s diverticulum (B) → vitelline duct remnant, bleeding + abdominal pain.

• Hirschsprung (A) → neural crest failure, constipation.

• Duodenal atresia (C) → double bubble, bilious vomiting.

• Omphalocele (D) → abdominal contents herniate into umbilical cord.

• Malrotation (E) → volvulus, obstruction.

A 67-year-old man develops a painful vesicular rash in a band-like distribution across the right thorax. This condition represents reactivation of latent virus in which of the following embryologically derived structures?

A. Epiblast
B. Hypoblast
C. Neural crest
D. Sclerotome
E. Myotome

C

Shingles (herpes zoster) arises from reactivation of varicella-zoster virus within dorsal root ganglia, which are derived from neural crest cells.

• E. Myotomes → muscles, not sites of viral latency.

• D. Sclerotomes → vertebrae.

• A, B. Epiblast and hypoblast → early embryonic germ layers, not ganglia.