Spermatogenesis and Oogenesis

spermatogenesis

• the process of sperm production in the seminiferous tubules of the testes

• occurs continuously after puberty

• cells become haploid gametes (sperm refers to mature spermatozoa)

where does spermatogenesis occur?

• occurs within the seminiferous tubules of the testes

• the development of sperm is supported by:

  • leydig cells (interstitial space between tubules)

  • sertoli cells (within the tubules surrounding the lumen)

spermatogonia to primary spermatocyte

• spermatogonia are the ‘germ cell’ of the male reproducitive system

  • they are cells from which gametes will develop

• there are two types:

  • type A: maintain the germ cell population

  • type B: differentiate into primary spermatocyte

from primary spermatocytes to spermatids

• type B spermatogonia become primary spermatocytes

• undergo meiosis 1 → two secondary spermatocytes

• meiosis 2 → four spermatids (haploid)

• this process takes approximately 26 days

from spermatids to spermatozoa

• spermiogenesis: the physical development of spermatid to spermatozoa (approximately 21 day process)

• spermatids → spermatozoa:

  • tail formation

  • acrosome development

  • nuclear condensation

• released into the lumen of the tubule → sperm at this sate are still non-motile

from the tubules to the epididymis

• spermatozoa are transported to the epididymis where the mature functionally

• spermatozoa at this stage are still non-motile

• movement is assisted by:

  • fluid secreted by sertoli cells, which helps push immature sperm along the ducts

  • peristaltic contractions of smooth muscle in the tubules also help propel the sperm

  • ciliated epithelial cells lining the ducts assist by sweeping the sperm toward the epididymis motility and ability to fertilise

maturation in the epididymis

• one in the epididymis, immature spermatozoa begin to mature

• this is due to environmental differences between the epididymis and the seminiferous tubules (i.e ion composition, proteins and osmotic concentration)

• sperm develop:

  • the ability to swim

  • the potential to react with the ova (fertilisation)

• this process takes place over 12 days, starting in the head of the epididymis, at the end mature sperm are stored in the tail, awaiting ejaculation ( or after approx 3 weeks of degeneration)

structure of mature sperm

• head:

  • nucleus → contains n chromosomes

  • acrosome → contains digestive enzymes to aid fertilisation

• body:

  • packed with mitochondria, generating ATP for tail movement

• tail:

  • flagellum that ‘whips’ enabling sperm motility

the role of mitosis in producing sperm

• spermatogonia are the germ cells at the base of the seminiferous tubule

• mitosis produces more spermatogonia

• this ensures a continuous supply of sperm

• type A spermatogonia can under mitosis and differentiate

• type B spermatogonia can only undergo meiosis becoming primary spermatocyte → spermatozoa

hormonal regulation of spermatogenesis

• the production (and rate of production) of sperm is regulated by the endocrine system

• the two gonadotropins (gonad-targeting hormones) target the cells in the gonads

  • these cells have specific receptors for these hormones

• LH secretion causes Leydig cells to secrete testosterone

• FSH and testosterone both target Sertoli cells

  • FSH encourages mitosis and the secretion of androgen binding protein

  • testosterone encourages meiosis and the development and maturation of sperm

the role of LH in spermatogenesis

• LH activates leydig cells that secrete testosterone

• testosterone is crucial for spermatogenesis as it triggers meiosis that leads to spermatozoa

the role of FSH in spermatogenesis

• FSH acts of sertoli cells within the seminiferous tubules of the testes

• FSH stimulates Type A spermatogonia to undergo mitosis

• this maintains the supply of spermatogonia for sperm production

role of testosterone in spermatogenesis

• testosterone is secreted by interstitial (leydig) cells in response to LH

• spermatogonia respond to testosterone differentiating into primary spermatocytes

• primary spermatocytes undergo meiosis to eventually become spermatozoa

when does oogenesis begin?

• before birth

• primordial germ cells become oogonia, which proliferate by mitosis to produce approx. 2 million oogonia

• al oogonia begin meiosis while the female is still in utero

• meiosis 1 pauses at prophase 1 before birth → recommences at puberty

• at this stage, the developing gamete is called the primary oocyte

• this part of the process is not regulated by hormones (gonadotropin independent)

primary oocyte to secondary oocyte

• occurs from puberty to menopause, once every 28 days

• one primary oocyte commences meiosis due to a surge in LH

• this produces:

  • secondary oocyte

  • one polar body (non functional)

• the secondary oocyte begins meiosis 2, before pausing at metaphase 2

• the secondary oocyte is ovulated (released from the ovary) and swept into the fallopian tube by fimbriae

• this process is facilitated by a follicle

secondary oocyte to ovum → the role of fertilisation

• the secondary oocyte is swept into the fallopian tube by the fimbriae and propelled along by cilia toward the ampulla (the typical site of fertilisation)

• the secondary oocyte is viable for 12-24 hours

• during this time, if the secondary oocyte is fertilised meiosis 2 will recommence → this will take place as the fertilisation is beginning

• meiosis 2 will complete producing:

  • one mature ovum

  • a second polar body (non functional)

• if not fertilised the secondary oocyte will degenerate and be expelled during menstruation

what is a follicle

• a strucutre that aids in the development and ovulaton of oocyte in the ovary

• every primary oocyte is contained within a follicle

• a follicle is sensitive to LH and FSH