Gametogenesis

Scope Gonadogenesis Primordial germ cells Ambisexual (indifferent ) gonad Gametogenesis Oogenesis Spermatogenesis

Ambisexual (indifferent ) gonad Primordial germ cells Originate within primary ectoderm Appear in yolk sac at 3 i.u.w Migrate dorsally to genital ridge at 5 i.u.w

4 Primitive sex cords Dev from cells of mesonephros & coelomic epith They proliferate to form genital ridges Ambisexual gonad M & F structural elements derived from 7 i.u.w TDF encoded on SRY instigates M; absence F Gonadogenesis…

Gametogenesis: Timing Males Gametes arise from spermatogonia Primordial germ cells remain dormant from 6 i.u.w to puberty Spermatozoa produced at puberty till death Females Primordial germ cells undergo few more mitotic divisions, begin meiosis 5 i.u.m Primary oocytes remain arrested in Prophase I until puberty

Gametogenesis… Prepares sex cells for MEIOSIS MI - chromosome no haploid and shape altered MII similar to mitosis but has haploid chrom. no Provides constancy of chromosome no from generation to generation Allows random assortment of maternal & paternal chromosomes btwn gametes Allows crossing over of chromosome segments

Gametogenesis Scheme of the normal maturation of divisions of germ cells

Spermatogenesis After migration, primordial cells are incorporated into primitive sex cords At birth sex cords contain 2 primordial cell types Spermatogonia arise from mesenchyme Sertoli cells within the genital ridge Lumina develop in sex cords - seminiferous tubules Begins at puberty - continues throughout life

Testis

Differentiation of spermatogonia 2 cell types - Type A & B

Spermiogenesis Terminal stage of spermatogenesis Consists of differentiation of spermatid into spermatozoon As a result of meiosis 4 spermatids arise from each spermatocyte Changes in spermatid Nuclear chromatin condense Acrosome forms at anterior end Flagellum develops

Maturation of spermatid 9-14 days in epididymis Stored in distal part of epididymis Pass along by ciliary action & muscular activity of epididymis Seminal fluid is added Average ejaculate 3ml containing about 300 million sperms Actively motile after ejaculation at 2mm/min

Spermatozoa - cont… Male sperms about twice number of F Female sperms more resistant to acidic environment M sperms are speedy but have short life span, rarely more than 24 hr F sperms are slower but live longer than 72hr

Spermatozoa - cont… Alkaline secretions are kind to both M & F but are more favorable for the more numerous speedy M Vaginal secretions are acidic & remain so until very near ovulation, when the alkaline cervical secretions increase, thus making vagina less acidic for a short time

Oogenesis Oogonia (primordial germ cells) Arise from endodermal cells of yolk sac Migrate to cortex of ovary, become Primary oocyte (& follicle) Undergoes MI at 20 i.u.w & blocked till puberty Follicular cells become stratified, called granulosa cells Granulosa cells elaborate zona pellucida Primary oocyte (& follicle) becomes Secondary oocyte (& follicle)

Development of ovarian follicles Follicular cells derive from genital ridge Primordial follicle = a single layer of follicular cells covering oocyte Secondary follicle Follicular cells divide, add more cell layers Tertiary (Graafian) follicle Fluid-filled spaces appear, coalesce & form antrum Liquor folliculi fills antrum

Tertiary (Graafian) follicle - cont… Antrum splits into an outer stratum granulosum (membrana granulosa) & inner cumulus oophorus around oocyte Follicular cells of oophorus cumulus immediately adjacent to ooctye are called corona radiata Theca folliculi is CT around follicle & has An inner vascular layer theca interna (estrogen synth) An outer fibrous theca externa Dev. of ovarian follicles - cont…

Maturation of follicles & oocyte

Maturation of follicles and oocyte - cont…

Zona pellucida A clear glycoprotein btwn oocyte & follicular cells Facilitates & maintains the binding of sperms in fert Binding to sperms mediated by the legand protein ZP3 and receptors on sperm membrane Induces acrosomal reaction Remains around oocyte during ovulation, fertilization and cleavage Sheds off just before implantation

Selective atresia Number of primordial follicles & oocytes decrease with age e.g. Second trimester - several million follicles At birth ovary - btwn ¼m & 1m follicles At puberty - few hundred thousand follicles Reproductive lifetime - btwn follicles

Ovulation Rapid increase of liquor folliculi Ruptures at an avascular site & releases oocyte, 1 st polar body & surrounding oophorus Oocyte activated to complete MII following fertilization Menarche btwn 11 & 14yr; menopause, 45 & 50yr

Menstrual & ovarian cycles Under FSH & LH control Occurs 14 days before onset of next menstrual cycle If fertilization occurs, a corpus luteum persists If fertilization does not occur the ovum degenerates, it is shed & cycles recommence