ILS 2: Embryology-Conception

Primary oocytes are arrested at

Prophase I

After puberty, oocyte is arrested at

Metaphase II

Secondary oocytes are released during

ovulation

When do secondary oocytes complete Meiosis II?

After fertilization

when do spermatogonium start 1st meiotic division?

after puberty

total length of spermatogenesis

74 days

site of fertilization

ampulla of uterine tubes

ovaries produce

estrogen & progesterone

hormones that cause ovarian follicles to mature to a Graafian follicle

FSH & LH

what event causes primary oocyte to complete M1 and begin M2 as a secondary oocyte?

LH surge

Ovulation is driven by these hormones

Prostaglandins and LH [expel oocyte from mature follicle]

a yellow body is also known as

corpus luteum [follicle remaining after oocyte expelled during ovulation]

function of corpus luteum

secretes progesterone & some estrogen to help endometrium prepare for implantation

Before being able to fertilize oocyte, spermatozoa must under these 3 steps:

1. Capacitation

2. Acrosome rxn

3. Polyspermy block induced

What is capacitation?

epithelial interactions btwn sperm and mucosal surface of uterine tube [~7hrs], stripping sperm cells

What is the acrosome rxn?

occurs after binding to zona pellucida;

induced by zona proteins

release enzymes [acrosin] needed to penetrate zona pellucida

Phases of Fertilization [3]

1. Penetration of corona radiata

2. Penetration of zona pellucida

3. Fusion of oocyte & sperm cell membranes

What happens in slow block of polyspermy?

• mechanical block [hyaline layer] hardens

• internal SER stores of calcium ions diffuse thru oocyte membrane to promote fusion of egg & release cortical granules

• permanent!

16-cell structure of developing embryo is known as

Morulla / Early Blastocyst

What is the embryoblast?

inner cell mass → embryo

What is the trophoblast?

outer cell mass → placenta

When does an early blastocyst/morulla become a blastocyst?

when fluid penetrates zona pellucida into the inner cell mass

• fluid-filled cavity forms

• zona pellucida disappears and implantation begins

• ~day 5

Implantation occurs …

when?

where?

5-15 days after fertilization

posterior superior wall of uterus in endometrial layer

What is AVE?

Anterior Visceral Endoderm

• dorsal-ventral polarity established in embryo

Which cells form AVE?

Hypoblast cells

-migrate to cranial end of embryo

AVE cells are classifed as which germ layer?

endoderm

• secrete signals to specific cranial end of embryo

Where do primitive streak and node begin formation?

at caudal end

Gestational Age

• counted from 1st day of the last LMP (last mentrual period)

• more accurate

• accounts for 2 weeks leading up to fertilization

Embryologic Age (Conception)

day-day events that begin @ time of fertilization

How to calculate fetus due date?

LMP

• count back 3 calendar months

• add 1 yr & 7 days

What occurs in 2nd week of development?

bilaminar germ disc

• 2 cell masses → 2 cell types each

• 2 cell cavities

Trophoblast differentiates into which 2 layers?

Cytotrophoblast

Synctiotrophoblast

Cytotrophoblast

inner layer of mononucleated cells (mitosis)

Synctiotrophoblast

• outer multinucleated zone WITHOUT distinct cell boundaries (no mitosis)

• comes from cells in cytotrophoblast

• invades maternal endometrium & becomes placenta

• makes hCG

Embryoblast differentiates into which 2 layers?

Hypoblast

Epiblast

Hypoblast

small cuboidal cells adjacent to blastocyst activity

Epiblast

tall columnar cells adjacent to amniotic cavity

• differentiate into amnioblast cells & line amniotic cavity [secrete amniotic fluid]

What is the exocoelomic (Heuser) membrane?

• hypoblast cells differentiate to form this thin membrane

  • lines inner surface of cytotrophoblast

    • AKA exocoelomic cavity AKA primitive yolk sac

Lacunae

blood network formed by syncytiotrophoblasts to form utero-placental circulation

penetration opening in surface epithelium of uterus [from implantation] is sealed by what?

fibrin coagulum/plug

hCG produced by syncytiotrophoblasts —> ?

stimulate progesterone production by corpus luteum

*maintains pregnancy until WEEK 8, when placenta takes over

hCG levels detected in maternal blood at day __

maternal urine at day __

8

10

low levels of hCG suggest

• spontaneous abortions

• ectopic pregnancy

high levels of hCG suggest

• pregnancy with multiples

• hydatidiform mole

• gestational neoplasias & tumors

Primitive yolk sac cells form

extraembryonic mesoderm

extraembryonic mesoderm is located between which 2 things?

1. inner surface of cytotrophoblast

2. outer surface of exocoelomic cavity

Extraembryonic cavity AKA chorionic cavity

large space between extraembryonic somatic mesoderm & extraembryonic splanchnic mesoderm

Secondary yolk sac functions [3]

• supplies temporary nutrition until placenta forms fully

  • disappears at end of 1st trimester

• Hematopoeisis

• site of primordial germ cells [gametes]

*entirely lined w/ endoderm

Primary Chorionic Villi

1st stage in placenta development

Chorion forms from

• extraembryonic somatic mesoderm

• 2 layers of trophoblast

gastrulation —> these 3 germ layers

1. ectoderm

2. mesoderm

3. endoderm

Gastrulation events

1. Primitive streak forms

2. Primitive node forms

3. Epiblast cells migrate to & thru primitive groove/streak [invagination]

   1. influenced by growth factors

Which cell type creates all germ layers via gastrulation?

epiblast

• endoderm: from displacing hypoblast

• mesoderm: cells lie btwn epiblast and endoderm

• ectoderm: cells remaining in epiblast

Primitive streak makes which germ layer?

mesoderm

• until week 4, then diminishes & remains in sacrococcygeal region

Notochord formed via

invagination of prenotochordal cells thru the primitive node

*mesoderm

Mesenchymal cells

pluripotent; can become any cell/tissue

Key events at Day 21 [end of week 3] in development?

• Heart begins to beat

• utero-placental circulation functional

Ectoderm gives rise to

• Skin epidermis

• Nervous system [CNS, PNS]

  • CNS from neural tube

  • PNS from neural crest

Mesoderm gives rise to

• Vascular system

• Urogenital system

• Muscles, bones, skin dermis

Endoderm gives rise to

• epithelial lining of

  • GI tract

  • Respiratory tract

  • urinary bladder

• glands

  • thyroid, parathyroid, liver, pancreas

Teratogen

known environmental, drug, or lifestyle factor that leads to birth defect

Which week of dvlpment is highly sensitive to teratogens?

Week 3

Malformation

complete or partial absence of a structure

OR

alterations of its normal configuration

Ex. Neural tube defects → anencephaly or spina bifida

Disruption

morphological alteration of an already formed structure by a destructive process

Ex. Amniotic bands produce vascular accidents → cleft lip, toe/finger amputations

Deformation

mechanical forces mold a part of the fetus over time

*often musculoskeletal

Ex. Club feet or hand from compression in amniotic cavity by oligohydramnios [too little amniotic fluid]

Syndrome

group of anomalies occuring together that have a specific common cause

• risk of recurrence known

Ex. Down Syndrome

Teratogenic Exposures

1. Infectious agents [TORCH]

2. Hyperthermia

3. Radiation

4. Chemical agents

   1. Thalomide, Vitamin A, Alcohol

5. Hormones

6. Maternal disease

7. Nutritional deficiences

8. Obesity

9. Heavy metals

Thalidomide

sedative linked to limb defects in 1961

Ex. Phocomelia - loss of limb’s long bones

Zika virus

infectious agent -→ microcephaly/small cranium in infants at early gestation

Fetal Alcohol Syndrome

• reduced growth

• small cranium

• indistinct philtrum

• thin upper lip

• depressed nasal bridge

• short nose

• flat midface

Neural Tube Deficiency [NTD]

Folate deficiency

• partial failure or reversal of neural tube closure

• → severe spina bifida, or anencephaly if in cranial region

Hydatidiform Moles

• fertilization of an oocyte lacking a nucleus

  • followed by duplication of male chromosomes to restore diploid number

• trophoblastic development can still occur and lead to placental tissue formation

• secrete high levels of hCG

  • may produce benign or malignant tumors

• pregnancy must be terminated

Ectopic pregnancy

blastocyst implants outside of body of uterine cavity

*ampulla most common site of ectopic pregnancy

frequently predisposed by chronic pelvic inflammation [PID]

• IVF, smoking, and tubal damage/surgery also risk factors