embryology

Question 1

https://anatomy.elpaso.ttuhsc.edu/embryo/question_menu.htm

Question 2

weeks of embryo development overview

Answer 2

Week 1: Fertilization → zygote → blastocyst formation

Week 2: Implantation; bilaminar disc (epiblast + hypoblast)

Week 3: Gastrulation → trilaminar disc (ectoderm, mesoderm, endoderm)

Weeks 3–8: Organogenesis (most susceptible to teratogens)

Week 4: Neural tube closes, heart begins to beat

Week 8: All major organs formed

Week 9+: Fetal period – growth and maturation

Question 3

3 germ layers of embryology and what structures are derived

Answer 3

ectoderm: Skin, hair, nails, CNS (from neuroectoderm), lens, enamel

mesoderm: Muscle, bone, blood, heart, urogenital system, adrenal cortex

endoderm: Gut lining, liver, pancreas, lungs (alveoli), thyroid

Question 4

what is derived from neural crest cells?

Answer 4

“MOTEL PASSES”

Melanocytes, Odontoblasts, Tracheal cartilage, Enterochromaffin cells,

Laryngeal cartilage, PNS, Adrenal medulla, Schwann cells, Spiral septum

Question 5

origins of the following structures

adrenal medulla
adrenal cortex
anterior pituitary
posterior pituitary
heart and blood vessels
GI tract lining
gonads

Answer 5

adrenal medulla: neural crest

adrenal cortex: mesoderm

anterior pituitary: oral ectoderm (rathke’s pouch)

posterior pituitary: neuroectoderm

heart and blood vessels: mesoderm

GI tract lining: endoderm

gonads: intermediate mesoderm

Question 6

body folding in embryo (2)

Answer 6

Lateral folding: Forms the body wall → closes ventral body cavity

Cephalocaudal folding: Brings heart and diaphragm into position

Question 7

fetal circulation shunts (3)

Answer 7

Ductus venosus: Umbilical vein → IVC (bypasses liver)

Foramen ovale: R atrium → L atrium (bypasses lungs)

Ductus arteriosus: Pulmonary artery → aorta (bypasses lungs)

Question 8

spermatogenesis steps

Answer 8

Spermatogenesis Steps:
1. Spermatogonia (diploid, 2n) — stem cells located in the basal layer of the seminiferous tubules undergo mitosis to maintain the stem cell pool and produce primary spermatocytes.

2. Primary spermatocytes (diploid, 2n) — enter meiosis I, where homologous chromosomes separate.
3. Secondary spermatocytes (haploid, n) — result from meiosis I; these quickly enter meiosis II.
4. Spermatids (haploid, n) — result from meiosis II; immature sperm cells with single sets of chromosomes.
5. Spermiogenesis — transformation of spermatids into mature, motile spermatozoa (loss of cytoplasm, development of tail, condensation of nucleus).
6. Spermatozoa — mature sperm released into the lumen of seminiferous tubules and transported to the epididymis for further maturation and storage.

spermatogonia –> primary spermatocyte –> first meiotic division –> two secondary spermatocytes –> second meiotic division –> four spermatids –> four sperm or spermatozoa

Question 9

steps of oogenesis

Answer 9

1. Oogonia (diploid, 2n)
   Multiply by mitosis during fetal development.
2. Primary oocytes (diploid, 2n)
   Enter meiosis I during fetal life but arrest at prophase I until puberty.
3. At puberty, each menstrual cycle:
   A primary oocyte completes meiosis I, producing:
   Secondary oocyte (haploid, n) (large cell)
   First polar body (small, usually degenerates)
4. Secondary oocyte begins meiosis II but arrests at metaphase II until fertilization.
5. If fertilization occurs:
   Secondary oocyte completes meiosis II, producing:
   Ovum (haploid, n) (mature egg)
   Second polar body
6. If no fertilization:
   Secondary oocyte degenerates.

primary oocyte in primary follicle –> first meiotic division –> secondary oocyte in mature follicle and first polar body –> second meiotic division –> mature oocyte and second polar body

Question 10

phases of the menstural cycle

Answer 10

Menstrual Phase (Days 1–5):
- Shedding of the functional layer of the endometrium → menstrual bleeding.

Follicular (Proliferative) Phase (Days 6–14):
- FSH stimulates growth of ovarian follicles.
- Developing follicles produce estrogen → rebuilds the endometrial lining.
- Estrogen peaks → triggers LH surge around day 14.

Ovulation (Day 14):
- LH surge causes release of the secondary oocyte from dominant follicle.

Luteal (Secretory) Phase (Days 15–28):
- Corpus luteum forms → secretes progesterone (and some estrogen).
- Progesterone maintains and thickens endometrium, preparing for implantation.
- If no fertilization → corpus luteum degenerates → drop in progesterone and estrogen → menstruation starts.

Question 11

hormones of mensutaral cycle

Answer 11

FSH- from anterior pituitary; stimulate follicle growth

LH- from anterior pituitary; triggers ovulation and corpus luteum formation

estrogen- from developing follicles; proliferation of endometrium

progesterone- from corpus luteum; maintains secretory endometrium (will degrade to corpus albicans if not fertilized)

Question 12

when does hCG take control to prevent corpus luteum from degeneration when fertilized

Answer 12

wk 20

Question 13

fertilization

enzyme?

Answer 13

sperms acrosome; acrosome reaction

digests zona pellucida of ovum via hyaluroniadse released from the acrosome

sperm penetrates zona pellucida in zona reaction and egg becomes impermeable to another sperm

oocyte arrested in metaphase; completes second meiotic division to become mature oocyte with a second polar body

nucleus of mature oocyte becomes female proneuclaus and sperm also becomes a pronucleus and its tail degenerates

membranes of 2 pronuclei break down and chromosomes arrange for mitotic division/ 23 chromosomes in each pronucleus combine- zygote of 46 chromosomes

cleavage; to get blastomere

Question 14

what happens to the oocyte when its fertilized by sperm

Answer 14

it was arrested in metaphase; so it completes its second meiotic division to become a mature oocyte with a second polar body

Question 15

sperm fertilizes egg

Answer 15

🧬 Steps of Egg Fertilization:
1. Sperm Capacitation
Sperm undergoes biochemical changes in the female reproductive tract, gaining the ability to penetrate the egg.

2. Acrosome Reaction
   Enzymes from the sperm’s acrosome digest the zona pellucida (glycoprotein shell) of the oocyte.
3. Sperm Penetration & Fusion
   One sperm fuses with the oocyte membrane; its nucleus enters the oocyte.
4. Cortical Reaction (Block to Polyspermy)
   Oocyte releases enzymes that harden the zona pellucida, preventing entry of additional sperm.
5. Completion of Meiosis II
   The secondary oocyte, previously arrested at metaphase II, completes meiosis → forms ovum + second polar body.
6. Pronuclear Fusion (Syngamy)
   Male and female pronuclei fuse → form a diploid zygote (2n).
7. Zygote Formation & Cleavage Begins
   The zygote starts mitotic divisions (cleavage), beginning embryonic development.