Gametogenesis

Scope

Gonadogenesis Primordial germ cells Ambisexual (indifferent ) gonad

Gametogenesis Oogenesis Spermatogenesis

Ambisexual (indifferent ) gonad

Primordial germ cellsOriginate within primary ectoderm Appear in yolk sac at 3 i.u.wMigrate 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

_______________________________________________________________________________________________________

Oogenesis and its Timing Spermatogenesis and its Timing

Primary oocyte Primary spermatocyte (44 + XX) (44 + XY)

Secondary oocyte (22 + X) Secondary spermatocyte (22+X or Y)

Polar bodies (22 + X) Spermatids (22 + X or Y)

_______________________________________________________________________________________________________

Oogonia (44 + 2X)

Spermatogonia (44 + XY)

Mitosis

Meiosis I (Reduction Division)

Meiosis II (Equational Division)

Spermatozoon 22 + X or Y

Mature ovum 22 + X

Spermiogenesis

Primordial cells dormant from 6 wks until puberty

from 6 i.u.w - puberty]

Enters MI at 20 wks arrested in prophase I until puberty

from 6 i.u.w - puberty]

Puberty to ± 45 yrs Puberty

to death

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 spermatogonia2 cell types - Type A & B

______________________________________________________________________________________________ 1. Dark Type A (Ad)

(Reserve Stem Cells, RSC)

2. Pale Type A (Ap)

50% 50%

3. Type B

(Differentiating Progenitors) Primary Spermatocytes (2N) Secondary Spermatocytes (1N)

Spermatids (1N)

Spermatozoa

______________________________________________________________________________________________

Quiescent and held in reserve. They go into cycle if depleted

Largely remain undifferentiated. Serve as Unipotential Stem Cells capable of mitotic proliferation 50% remain undifferentiated as RSC

Largest cells. Undergo the lengthy prophase of MI (maturation division)

Seldom seen. Rapidly enter meiotic II (MII) division

Haploid in number and small. Spermatids transform without division, a process called spermiogenesis

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 & 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 300 - 400

follicles

Ovulation

Rapid increase of liquor folliculi Ruptures at an avascular site & releases

oocyte, 1st 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