Gr 12 Gameteogenesis

Spermatogenesis and oogenesis both involve mitosis, cell growth, two divisions of meiosis and differentiation
Processes in spermatogenesis and oogenesis result in different numbers of gametes with different amounts of cytoplasm

Spermatogenesis

Gametogenesis is the process by which diploid precursor cells undergo meiotic division to become haploid gametes (sex cells)

In males, this process is called spermatogenesis and produce spermatozoa (sperm)
in the male, spermatogenesis occurs from puberty to old age, producing immense numbers of spermatozoa at an average rate of 1.5 million spermatozoa per minute.

Sperm production takes place inside tubules in the testes. Here, diploid cells divide by mitosis to produce numerous diploid spermatogonia, which grow to form diploid primary spermatocytes. The first division of meiosis then takes place, forming two haploid secondary spermatocytes. The second division of meiosis then produces haploid spermatids, which mature into spermatozoa.

Spermatogenesis describes the producton of spermatozoa (sperm) in the seminiferous tubules of the testes
The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis
These cells (spermatogonia) then undergo a period of cell growth, becoming spermatocytes
The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (spermatids)
The spermatids then undertake a process of differentiation in order to become functional sperm cells (spermatozoa)

Oogenesis

In females oogenesis begins in early foetal life. All oocytes ever to be formed in females are produced during foetal life. Many of them degenerate with time and at birth the ovaries contain about 2 million oocytes. All the oocytes go into meiotic arrest when they reach the first meiotic division during foetal life. The primary oocytes remain in the prophase of the first meiotic division until the time of puberty, when they are gradually released to complete meiosis at regular intervals known as the ovarian cycle. On the average only one oocyte matures during each cycle, which occurs at approximately monthly intervals, so that the total amount of oocytes to be ovulated is about 500 oocytes in a lifetime.

Oogenesis describes the production of female gametes (ova) within the ovaries (and, to a lesser extent, the oviduct)

The process begins during foetal development, when a large number of primordial cells are formed by mitosis (~40,000)
These cells (oogonia) undergo cell growth until they are large enough to undergo meiosis (becoming primary oocytes)
The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles
The primary oocytes remain arrested in prophase I until puberty, when a girl begins her menstrual cycle
Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes
These cells will complete the first meiotic division to form two cells of unequal size
One cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body
The polar body remains trapped within the follicle until it eventually degenerates
The secondary oocyte begins the second meiotic division but is arrested in metaphase II
The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube)
The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte
If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body)
Once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

Oogenesis follows a similar pattern, but many fewer gametes are made than during spermatogenesis, and the process takes much longer, with long ʻwaiting stagesʼ. It takes place inside the ovaries, where diploid cells divide by mitosis to produce many oogonia. These begin to divide by meiosis, but stop when they reach prophase I. At this stage, they are called primary oocytes, and they are, of course, still diploid. All of this happens before a baby girl is born, and at birth she has around 400 000 primary oocytes in her ovaries. When she reaches puberty, some of the primary oocytes get a little further with their division by meiosis. They proceed from prophase I to the end of the first meiotic division, forming two haploid cells. However, the division is uneven; one cell gets most of the cytoplasm, and becomes a secondary oocyte, while the other is little more than a nucleus, and is called a polar body. The polar body can be thought of as simply a way of getting rid of half of the chromosomes, and has no further role to play in reproduction.

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Each month, one secondary oocyte is released into the oviduct from one of the ovaries. If it is fertilised, it continues its division by meiosis, and can now be called an ovum. The chromosomes of the spermatozoan and the ovum join together to form a single diploid nucleus, and the cell that is made by this process is called a zygote. The zygote can now divide repeatedly by mitosis to form first an embryo, and then a fetus.

Comparing Gametogenesis

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