Session 3: Gametogenesis Flashcards
After colonising the gonad, what will germ cells do?
Proliferate by mitosis Reshuffle genetically and reduce to haploid by meiosis Cytodifferentiate into mature gametes
How does genetic variation occur?
Crossing over of homologous chromosomes (recombination Independent assortment Random segregation
Differences between gametogenesis in males and females.
Females produce a finite number of gametes (400 ova in a lifetime). This means that females have 400 chances to reproduce. The production of ova is not continuous but intermittent. Males produces a vast amount of sperm and it is continuous. This means that males remain fertile much longer than females and can reproduce whenever.
Where does spermatogenesis take place?
In the seminiferous tubules of the testes.
Explain the initial part of spermatogenesis.
Spermatogonia which are germ cells can be found in the basal compartment of the seminiferous tubules. Spermatogonia divide by mitosis to give two primary spermatocytes. One spermatocyte will replace the spermatogonia, the other spermatocyte will undergo meiosis I to produce two secondary spermatocytes. The secondary spermatocytes will undergo meiosis II to produce 2 spermatids each and a total of 4 spermatids.
What is spermatogonia divided into?
Ad spermatogonium Ap spermatogonium
What is the spermatogenic cycle?
Defined as time taken for reappearance of the same stage within a given segment of tubule. This is usually 16 days in humans. Aka the time it takes for spermatids at the same stage in the cycle to show up again in the same stage.
What is the spermatogenic wave?
Distance between groups of spermatids at the same level of maturation. The distance down the tubule where you meet another cell in the same stage.
What is the next stage once 4 spermatids have been formed?
Spermiogenesis.
Explain spermiogenesis.
The spermatids are released into the lumen of the seminiferous tubules through a process called spermiation.
At this stage they are still non-motile and the transport through seminiferous tubule, through the rete testis and ductuli efferentes is facilitated by sertoli cell secretions called peristaltic contractions until they reach epididymis.
During this journey from the seminiferous tubules to the epididymis the spermatids mature into spermatozoa. The spermatozoa are fully motile.
Explain the structure of the sperm.
Head with a nucleus and its genetic information that will fuse with the oocyte. There is an acrosome layer on the head as well which allow this to happen.
There is a tail which provides motility for the sperm and it has a lot of mitochondria for producing ATP.
How much of the ejaculate does the sperm make up?
2-5%
What is the rest of the ejaculate?
70% seminal vesicle secretions
25% secretions of the prostate
<1% secretions from the Bulbourethral gland.
Purpose of the seminal vesicle secretions.
Support the sperm by giving amino acids, citrate, fructose and prostaglandins.
Fructose is the main energy source for sperms.
Purpose of the secretion of the prostate.
Proteolytic enzymes and zinc. Zinc controls the motility.
Explain the maturation of oocytes before birth.
Germ cells colonise the gonadal cortex and differentiate into oogonia.
The oogonia will start to proliferate rapidly by mitosis and by the end of 3rd month the oogonia will have arranged in clusters surrounded by flat epithelial cells.
The majority of the oogonia will continue to divide by mitosis, however some will enter meiosis. The once that enter meiosis will stop in prophase of meosis I. These oogonia are now known as primary oocytes.
Cell death starts to occur and many oogonia and primary oocytes start to degenerate. This is called atresia and happens to make sure that only the good primary oocytes are left. By month 7 majority of oogonia have degenerated. The surviving 2 million primary oocytes have now all entered meiosis I.
The primary oocytes will remain like this until puberty and at birth these primary oocytes will be surrounded by follicular cells. The primary oocytes with their follicular cells are now called primordial follicles.
Explain the preantral phase.
The follicular cells of the primordial follicles will start to grow and form granulosa cells. There is a change from flat to cuboidal to stratified epithelium. The granulosa cells will secrete a glycoprotein as well called the zona pellucida.
This is now called a primary follicle.
Explain the antral phase.
Development of the primary follicle continues and fluid filled spaces appear between the granulosa cells. This becomes a collective space and is termed the antrum.
The follicle itself is now called a secondary follicle.
The outer fibrous layer develops into the theca interna and the theca externa.
Explain the preovulatory phase.
LH starts to surge (leutinizing hormone).
This causes the secondary follicle to complete meiosis I and produce 2 haploid cells. The majority of the cytoplasm of the 2 haploid cells will go to one of the daughter cells. The cell that won’t receive much cytoplasm is called a polar body.
This happens to all of these 15-20 secondary follicles but only one is chosen to mature fully. This chosen one is called the Graafian follicle.
The Graafian follicle will enter meiosis II but will arrest in metaphase before ovulation. Meisosis II will only be completed if the oocyte is fertilised. It will otherwise degenerate.
(Cells can stay in meosis II for a long period of time (up to 40 years) which may explain why as a woman gets older there is more of a risk of mutation and trisomy)
What is the lifespan of the corpus luteum?
It remains constant and will degenerate after 14 days if no fertilisation occur to become the corpus albicans. Corpus albicans is a mass of scar tissue.
Explain oocyte transport.
Shortly after ovulation when the oocyte has broken free the oocyte will be taken up by fimbriae. The fallopian tube will begin to contract rhythmically and the oocyte is carried into the tube by sweeping movements of the fimbriae and by motion of cilia.
Oocyte is then propelled forwards by peristaltic muscular contractions of the tube (compare with male spermiogenesis) and by cilia in the mucosa.
