Gametogenesis….. Geoffrey T. Meyer Room 217, School of Anatomy and Human Biology Coordinator of Human Organs and Systems 214 Semester 2

One Essential Question: Why is sexual reproduction necessary? The main purpose of sexual reproduction is the formation of offspring who are genetically different from one another and from their parents. Meiosis is the fundamental process underlying sexual reproduction. It involves two essential outcomes: 1. Reduction Division - the process in which each gamete receives a haploid set (n) of chromosomes and genes. The diploid number (2n) is restored on fusion of two gametes. 2. Rearrangement of genes on the maternal and paternal chromosomes. This ensures that the offspring are genetically different from one another.

Consequences of meiosis Meiosis Sperm and oocytes contain only one representative of each homologous pair of chromosomes Maternal and paternal chromosomes are randomly assorted Crossing over increases genetic variation

Seminiferous epithelium Testis Seminiferous epithelium Spermatogonia Diploid spermatogenic cells Location? Destiny? Derived from? Implications for male fertility

Undergoes 1st meiotic division Secondary spermatocytes Primary spermatocyte Undergoes 1st meiotic division Secondary spermatocytes Spermatids

Seminiferous epithelium Testis Seminiferous epithelium Spermatogonia Diploid Primary spermatocytes (meiosis I) Secondary spermatocytes (meiosis II) Spermatids Haploid

Spermatogonia stem cell Meiosis in the male Spermatogonia stem cell Puberty until death

Meiosis in the female

Oogonia Meiosis in the female Proliferation of oogonia (equivalent of spermatogonia in the male) occurs in the fetal ovary Birth Ovulation Fertilisation Polar bodies arise from unequal cell division

7 million primary oocytes 400,000 at birth Several primary follicles initiate maturation 1 completes its development Remainder degenerate (atresia) 400-500 ovulate Atretic follicles

Gametes don’t develop on their own – they need supporting cells So I wish to introduce the: Sertoli cell (testis) Interstitial cell of the testis (Leydig cell) Granulosa cell (ovary)

Sertoli cells Location Junctional complexes Divide the seminiferous tubule into a: (a) basal compartment (b) adluminal compartment Support/protect/nourish spermatogenic cells Phagocytic Release mature spermatids Secrete ABP (FSH), Inhibin (-ve; FSH) and Activin (+ve; FSH)

Leydig cells Location Secrete Testosterone (LH) requiring Prolactin (LH receptor)

Corpus luteum formation Theca interna Theca interna Theca lutein Granulosa cells Granulosa cells Granulosa lutein

Granulosa lutein cells (FSH) Theca lutein cells (LH) Theca interna Ovulation Androstenedione (LH) Corpus luteum Granulosa cells Progesterone Estradiol Testosterone Estradiol (FSH) Granulosa lutein cells (FSH) Theca lutein cells (LH) Acquire LH receptors