Ch. 20 Reproduction (Day 1) Flashcards
What are the 2 types of cells in the human life cycle?
- Germ cells
- -ova, sperm - Somatic cells
- -everything else in the body
Gametogenesis: Males vs. Females
Males: large numbers of gametes produced continuously from stem cells, beginning at puberty and extending until senescence (age-dependent decrease in testosterone production)
Females: release only ONE gamete at a time from limited pool of preformed gametes in a process that is repeated at regular monthly intervals
Where does gametogenesis begin?
In utero - mitotic divisions to increase germ cell numbers; PAUSES at birth and RESUMES at puberty
Timing varies between males and females
Spermatogenesis
- Spermatogonium DNA replicates but no division to form primary spermatocyte (46 chromosomes to 92) at PUBERTY
–MEOISIS 1 OCCURS–
- Primary spermatocyte splits into 2 secondary spermatocytes (46 chromosomes each)
–MEIOSIS 2 OCCURS–
- The 2 secondary spermatocytes split into 4 spermatids (23 chromosomes each), which mature into sperm via spermiogensis
- Sperm have 23 chromosomes each, released from tests during “spermiation”
Oogensis
- Oogonium (46 chromosomes) DNA replicates but no division into Primary Oocyte (92 chromosomes)
–MEIOSIS 1 OCCURS (puberty)–
- primary oocyte splits into 1 secondary oocyte (46 chromosomes) and 1 polar body (dies)
- secondary oocyte begins Meiosis 2 and is released during ovulation
- secondary oocyte splits into 1 Ootid (23 chromosomes) and 1 polar body (23 chromosomes, dies)
5a. Meiosis 2 is completed ONLY IF FERTILIZATION OCCURS
5b. If fertilization doesn’t occur, Ootid dies (?)
Oocyte production
- Primary oocyte
- a) toward end of gestation, female’s oogonia begin meiosis to produce PRIMARY OOCYTES
- b) ovaries of newborn girl have 2 million primary oocytes
- c) by puberty, this number is cut to ~400,000
- d) only about 400 of these will be ovulated in her lifetime - Primary oocytes contained w/in primary follicles - have one layer of cells
- a) in response to FSH, some of primary follicles grow to produce many layers of granulosa cells (granulose cells support developing oocyte in middle of that follicle)
- b) some develop fluid-filled vesicles called secondary follicles - Continued growth results in fused vesicles to form single atrium; this is a mature Graafian follicle
- As Graafian follicle grows, primary oocyte finishes meiosis 1 to become secondary oocyte (plus a polar body, which soon degenerates)
- Secondary oocyte begins meiosis 2, but stops at metaphase 2
- Meiosis 2 will complete, ONLY if there is fertilization of ovum
Is it the sperm or egg that determines genetic sex of zygote?
Sperm
Sex determination in embryo
If zygote contains Y chromosome: male (even if multiple X’s)
If zygote gets only Y but no X –> lethal b/c X chromosome is essential for survival
X-inacitvation in females: early in development, after ovaries develop, one X in each body inactivates (becomes Barr body) - inactivation is random - some may be sperm-derived, others ovum-derived
Are polar bodies and Barr bodies the same?
No, they’re 2 different bodies
Why does X-inactivation occur in females (Barr body)?
So that only one X is viable in gamete (for reproduction later on in life)
random inactivation of one of X chromosomes
Chromosomal sex and development of embryonic gonads
Genetic sex is determined by which sex chromosome is carried by the sperm (X or Y)
Key gene carried on Y chromosome: “Testis Determining Factor” (TDF), aka “Sex Determining Factor” (SRY)
If sperm contributes Y: SRY expression stimulates testis differentiation
If sperm contributes X: lack of SRY allows ovary differentiation
The presence/absence of what gene on the sperm determines genetic sex?
SRY (aka TDF) gene
Presence of SRY = male
Absence of SRY = female
Sexual differentiation occurs early in development - internal organs
Regardless of genetic sex, embryo has potential to become phenotypically male or female - female pattern occurs unless humoral signals are released from fetal testis
Depends on presence of SRY gene on Y chromosome
If female: no SRY gene, biopotential gonads –> ovaries
If male: SRY expression elicits gonads –> testes
Testes produce masculinization factors (testosterone, anti-Müllerian hormone [AMH])
Wolffian Duct
Male
Degenerates if SRY isn’t present; develops if SRY is present
Müllerian Duct
Female
Degenerates if SRY is present; develops [ovaries, female reproductive organs] if SRY is NOT present
T/F: Testes secretion of testosterone is what determines if embryo is male or female.
FALSE
Testes secretion of testosterone is NOT what determines genetic sex - it’s the presence/absence of SRY gene expression that determines genetic sex
If SRY gene is present, testes develop and then testosterone is secreted
Leydig Cells
Males
Secrete testosterone
Controls development of Wolffian duct into accessory structures, development of external male genitalia (via DHT)
Sertoli Cells
Males
Secrete Anti-Müllerian hormone
Causes regression of Müllerian duct
T/F: Testosterone is NOT produced until testes differentiate
True