Where do primordial germ cells (PGCs) originate?
PGCs originate in the extraembryonic mesoderm of the yolk sac, immediately behind the primitive streak, at approximately 3 weeks of gestation in humans.
What are the three unique biological processes that PGCs undergo?
Lengthy migration through the developing embryo to the gonadal ridge;
Erasure of epigenetic information from the previous generation;
Reactivation of the X chromosome (Barr body) in XX cells
What is the SRY gene and what is its function?
SRY (Sex-determining Region Y) is the key switch gene in male sexual differentiation. It acts briefly in male embryos to initiate differentiation of Sertoli cells in the somatic cells of the genital ridge.
What is the default sexual differentiation pathway in mammals?
The female route of sexual development is the default differentiation pathway. This was demonstrated by Alfred Jost’s experiment showing that mammalian embryos castrated prior to testis differentiation develop phenotypically as females.
What is the function of Anti-Müllerian hormone (AMH/MIS)?
AMH/MIS is secreted by Sertoli cells and suppresses the default pathway that would develop Müllerian ducts as precursors of female sexual anatomy. It causes regression of the female Müllerian duct.
How long does spermatogenesis take in humans?
Spermatogenesis takes approximately 65 days in humans, from spermatogonium to spermatozoon.
What are the three phases of spermatogenesis?
Proliferation (mitotic expansion, spermatocytogenesis): spermatogonia
Reduction division (meiotic reduction): spermatocytes
Differentiation (morphological transformation, spermiogenesis): spermatids
What is the blood-testis barrier and how is it formed?
The blood-testis barrier is formed by tight junctions between Sertoli cells. It restricts passage of substances from blood to the lumen of seminiferous tubules and protects sperm from antibodies circulating in the bloodstream.
What replaces histones during spermiogenesis?
During spermiogenesis, somatic histones are replaced first by transition proteins (TNP2) and then by protamines (PRM1 and PRM2). This substitution is necessary for extremely compact packaging of sperm DNA.
What percentage of chromatin do mature human sperm retain in nucleosomal configuration?
Mature human sperm retain approximately 15-20% of their chromatin in a nucleosomal configuration (with histones).
What is the optimal temperature for spermatogenesis and why?
The optimal temperature is approximately 2°C below body temperature. The testes’ external position outside the body cavity achieves this lower temperature, which is necessary for proper spermatogenesis.
When do oogonia start to enter their first meiotic division?
Oogonia start to enter their first meiotic division around the twelfth week of gestation (end of first trimester) in humans.
At what stage do primary oocytes arrest and where do they remain arrested?
Primary oocytes arrest in the diplotene stage of prophase I (dictyate or germinal vesicle stage) after chromatid exchange and crossing-over. They remain arrested until recruited after puberty.
What is the maximum number of oocytes in human development and when is it reached?
The maximum is 6-8 million oogonia, reached at 16-20 weeks of gestation. This is reduced to approximately 1-2 million by birth.
What are the four proteins that make up the zona pellucida?
ZP1, ZP2, ZP3, and ZP4. These are expressed coordinately under control of the FIGα gene. ZP3 serves as the primary sperm receptor.
How long does the process of oogenesis take from primordial germ cell to preovulatory oocyte?
A minimum of 11 years. Human oocytes complete meiosis only after fertilization.
What are the two distinct populations of granulosa cells in antral follicles
Cumulus granulosa cells: surround the oocyte, mitotically inactive
Mural granulosa cells: line the follicle wall, become columnar and express LH receptors under FSH influence
When does the theca layer form during follicular development?
In humans, the theca layer does not form until the follicle contains between three and six layers of granulosa cells.
What percentage of follicles undergo atresia rather than ovulation?
Over 99% of follicles are destined to die through atresia rather than ovulate.
What is the timing sequence after the LH surge?
LH surge: 0 hours
+15 hours: GVBD
+20 hours: first meiotic metaphase
+35 hours: second meiotic metaphase (first meiosis complete)
+38 hours: ovulation
What are the two components of MPF (Maturation Promoting Factor)?
A 34-kDa protein (p34cdc2): serine/threonine kinase activated by dephosphorylation
Cyclin B: activated by phosphorylation, substrate for p39mos
What is the function of the cytoplasmic polyadenylation element (CPE)?
CPE controls the expression of stored mRNAs in oocytes. ACE-containing mRNAs are masked and protected from degradation by deadenylation, while non-ACE-containing mRNAs have long stable polyA tails for immediate translation
What is the relative composition of RNA types during oogenesis?
Approximately 60-65% rRNA, 20-25% tRNA, and 10-15% mRNA.
How many mitochondria does a mature oocyte contain?
Approximately 10⁵ (100,000) mitochondria. During oocyte growth, mitochondria increase in number and become more spherical with fewer concentrically arched cristae.
