Lecture 2 and 3: Introduction to Embryology and Sexual Differentiation

Question 1

Hypothalamic-pituitary-gonadal HPG Axis

Answer 1

surge and tonic centers in the hypothalamus send messages to the pituitary to release hormones and those hormones impact the activity and growth of the gonads

Question 2

Early Embryogenesis

Answer 2

step 1: highly specialized cells= sperm and oocyte
after fertilization, the zygote is formed and differentiation marker erased

Step 2: totipotent cells: cells capable to differentiate into any type of embyronic (body) or extra embryonic (placenta) cells

Step 3: at 16 cells (blastocyst) 1st round of differentiation into 2 types of cells
TYPE 1: trophectoderm: multipotent cells capable to differentiate into any type of cells within a cell lineage, in this case PLACENTAL cells

TYPE 2: inner cell mass (ICM): pluripotent cells: cell capable to differentiate into any type of cells in the embryo

Question 3

Gastrulation

Answer 3

2nd round of differentiation

primitive streak= origin of meso and endoderm

Hypoblast= primitive endoderm and will eventually disappear

Ectoderm, mesoderm, and endoderm and the 3 germ layers

endoderm= digestive system, lungs and endocrine system

mesoderm= muscle, skeleton, cardiovascular, most of reproductive system and urinary system

ectoderm= nervous system, skin, hair and external parts fo repro tract

Question 4

Primordial Germ Cells (PCGs)

Answer 4

PGCs are the founder for the germ line in animals. all their descendants are germ cells

germ line specification occur early in development through inductive signalling from extra-embryonic tissue to the epiblast during gastrulation

the PGCs migrate by amoeboid movement via the dorsal mesentery of the hindgut to the gonadal ridge

chemotaxis signalling produced by gonadal ridge coordinates the migration process

upon colonization of the gonadal ridge, primitive sex cords are formed

PGCs interact with gonadal somatic cells that determine their sex-specific commitment based on the gonadal environment

testis= spermatogonia stem cells

ovary= oogonia stem cells

Question 5

Testicular Descent Abnormalities

Answer 5

cryptorchidism

• failure of the testis to descend
• unilateral: fertile with testosterone
• Bilateral: sterile with testosterone

inguinal herniation
- intestine pass through the inguinal canal

Question 6

Sexual Differentiation of the Hypothalamus

Answer 6

first the cells in the neuroectoderm of the forebrain differentiate into hypothalamic neurons, forming multiple centers including two GnRH neurons centers (tonic and surge)

Question 7

Female Sexual Differentiation of the Hypothalamus

Answer 7

estradiol secreted by ovary cannot cross blood-brain-barrier due to binding with alpha-fetal protein, this prevents E2 entering the brain and so the surge center remains active

Question 8

Male Sexual Differentiation of the Hypothalamus

Answer 8

testosterone does ont bind to alpha fetal protein so it CAN cross the BBB and is converted to estradiol in the brain . estradiol in the brain deactivates surge center so males can only have activity in tonic center

Question 9

Freemartin Formation in Cattle

Answer 9

twins of distinct sex - one heifer and one bull

• 95% of heifers will be sterile
• bull is normal

chorions of the two placentas fuse, fetuses share blood supply and the heifer is exposed to testosterone from the male