Spermatogenesis/Oogenesis

GnRH

• secreted from hypothalamus

• stimulates gonadotroph cells in anterior pituitary (LH + FSH release)

LH

• secreted from anterior pituitary

• in male: stimulates Leydig cells in testicle to secrete testosterone

• in female: stimulates Theca cells

FSH

• secreted from anterior pituitary

• in male: stimulates Sertoli cells to encourage spermatogenesis

• in female: stimulates Granulosa cells

• inhibited by inhibin, promoted by activin

Estradiol/testosterone

• regulate behavior

• in male: spermatogenesis

• in female: cyclicity

Sertoli cells

• stimulated by FSH to encourage spermatogenesis

• release androgen binding protein in testes to increase testosterone concentration in seminiferous tubules

Overall function (male tract)

• manufacturing in testes (seminiferous tubules)

• fluid absorption & cell alterations in head/body of epididymus

• stored in tail of epididymus

• final alterations & packaging in accessory sex glands

• delivered via penis

Spermatogenesis

• process by which male gamete cells (spermatogonia) develop into spermatozoa

  • spermatocytogenesis

  • spermatidogenesis

  • spermiogenesis

• quiescent until puberty, continuous after

Seminiferous tubules

• exocrine testis (majority of parenchyma), site of spermatogenesis

• stratified epithelium

• maturation occurs inwards

Spermatogonia

• most immature male gamete

• either replicate to replenish proliferative pool or replicate to primary spermatocytes (spermatocytogenesis)

  • both mitosis!

Primary spermatocytes

• form from spermatogonia

• undergo first meiotic division

Secondary spermatocytes

• form from primary spermatocytes

• undergo second meiotic division (spermatidogenesis)

Spermatid

• form from secondary spermatocytes

• haploid cells

• undergo morphologic change (spermiogenesis)

Spermatozoa

• form from spermatid

• morphologic change from round cell to elongated cell with motile capabilities

• embedded “head” first in apical aspect of seminiferous tubule, tails in lumen

Spermiogenesis

• formation of head, neck, middle piece, principle piece, & tail

• loss of cytoplasm

Spermiogenesis phases

• golgi: golgi body forms proacrosomic granules, centrioles aggregate behind nucleus

• cap: acrosomic vesicle elongates, remaining golgi migrates behind nucleus

• acrosomal: acrosome covers nucleus, middle piece forms, cytoplasm decreases

• maturation: cytoplasm lost, mitochondria aggregate as “motor”

Spermatogenic cycle

• ~62 days in dog/cow

• ~55 days in horse

• effects of injury + reappearance of normal sperm delayed

Sex cord development

• mesothelial cells invaginate into gonadal ridges

• form plates/cords of tissue where oocyte precursors develop

Oogenesis

• pool of germ cells built during gestation, no longer replenished after birth

• fetal, prepubertal, postpubertal

Fetal oogenesis

• sex cord breakdown

• primordial germ cells from yolk sac become oogonia, undergo mitosis to build pool, & cluster as ovarian cortex

• arrest division in prophase of meiosis I as primary oocytes

Primordial follicles

• contains a primary oocyte surrounded by collagenous basal lamina & follicular cells

Atresia

• degeneration & loss of follicular structures & oocyte (fate of majority of cells)

Prepubertal oogenesis

• quiescent phase with atresia

Postpubertal oogenesis

• cyclical development of follicles begins

• first meiotic division is completed & secondary oocyte is formed (often at ovulation, some variation)

• second meiotic division occurs at fertilization

• majority of oocytes undergo atresia, small proportion ovulated & fertilized

Polar bodies

• formed as “left-overs” of oocyte meiosis

• typically 3 formed in the oogenesis process