Outcomes
- Analyze and describe the structure and function of female and male
mammalian reproductive systems (313-3)
- examine the processes of spermatogenesis and oogenesis
- explain why there is only one functional egg produced during oogenesis
- describe the structure of sperm and egg cells
Introduction
Gametogenesis is the creation of gametes by meiotic division of gametocytes into various gametes. The male form of gametogenesis is called spermatogenesis, while the female form is called oogenesis.
Spermatogenesis refers to the creation, or genesis, of spermatozoa, which occurs in the male gonads. Sperm cells develop initially from germ stem cells known as spermatogonia. As these differentiate they go through the following stages:
- spermatocytogenesis (creation of spermatocytes):
- mitosis of spermatogonia into primary spermatocytes
- meiosis of primary spermatocytes into haploid secondary spermatocytes
- spermatidogenesis (creation of spermatids through mitosis of secondary spermatocytes)
- spermiogenesis (creation of spermatozoa through further development of spermatids). The female counterpart to spermatogenesis is oogenesis.
The process of spermatogenesis is highly regulated via hormonal messengers and also the cell-cell communication between the developing germ cells (sperm cells) and the nurse-like Sertoli cells.
The Sertoli cell is essential for spermatogenesis as it provides support for the developing sperm cells - moving them towards the lumen of the seminiferous tubule as they develop until maturity when they are released. The Sertoli cell also reduces motility and capacitation(initiation of the acrosome reaction) of the sperm cells so viability is maintained.
Spermatozoa are produced in seminiferous tubules in the testes. They start off as spermatogonia, undergoing mitosis becoming a type A spermatogonium or a type B spermatogonium. Type B spermatogonia become primary spermatocytes. Primary spermatocytes go through a meiotic division to become secondary spermatocytes, which undergo another mit-otic division to become spermatids. Type A spermatogonia stay as spermatogonia, and do not change. They act as stem cells and will divide again producing more Type A and B cells.
The primary spermatocytes contain twice the DNA of a normal body cell (2 × 2N). Each primary spermatocyte divides into two secondary spermatocytes containing two sets of chromosomes (2 × 1N). The secondary spermatocytes then divide into two spermatids, each containing just one set of chromosomes (1N), half the DNA needed to make a human being. (The other half will come from the ovum at fertilisation).
The spermatids at this point are still round cells. During the process called spermiogenesis the sperm extrude tails, repackage DNA into a head, cap the head with a lysosome-like acrosome and construct an energy transducing neck region of tightly packed mitochondria to finally become mature testicular spermatozoa. They are then released into the lumen of the seminiferous tubule and travel with fluid from the testis into the epididymis for further maturation processes.
Morphological, physiological, and biochemical changes are among the maturation steps the sperm undergoes in the epididymis. During migration through the epididymis the sperm sheds excess cytoplasm, released as the cytoplasmic droplet. In transit, it also gains the capacity for motility. Finally, while traversing the epididymis the sperm's source of energy gradually changes from primarily glycolysis to oxidative respiration. By the time the mature sperm exits the epididymis, it is streamlined, motile, and has sufficient energy to draw near to an ovum and fertilize it.
Oogenesis is the creation of an ovum (egg cell) in the ovarian follicle of the ovary. Oogenesis is not to be confused with oocytogenesis, which is the formation of oocytes, which are the female gametocytes that divide later into ova.
In mammals, oogonial transformation into oocytes (oocytogenesis) is completed either before or shortly after birth. The further development comes to a rest during prophase I of meiosis until puberty. In Ascaris, the oocyte does not even begin meiosis until the sperm touches it.
In meiosis, the oocyte divides in four, of which only one becomes an ovum, the others becoming polar bodies. There may be two polar bodies, in which case one is diploid and did not undergo meiosis II.
Structure of the Sperm
Mature sperm, know formally as spermatozoa, have a morphology that most people over the age of ten would recognize immediately. The nucleus is contained within the head, which, for most mammals, has a flattened, oval shape. During spermiogenesis, the haploid sperm cell develops a tail or flagellum, and all of its mitochondria become aligned in a helix around the first part of the tail, forming the midpiece. The entire cell is, of course, enveloped by a plasma membrane. The image to the right shows these structures at the light microscopic level with a bull sperm.
The other structure in the mature sperm that plays a critical role in fertilization is the acrosome. The acrosome is, in essence, a gigantic lysosome that forms around the anterior portion of the nucleus. It is bounded by a membrane that is considered to have two faces - the inner acrosomal membrane faces the nucleus, while the outer acrosomal membrane is in close contact with the plasma membrane.
The image to the right shows the front end of a stallion sperm, viewed with an electron microscope. The ruffled appearance of the plasma membrane is an artifact of fixation. The acrosome is the dark band of material between the plasma membrane and nucleus - inner and outer acrosomal membranes are not clearly visible at this magnification.
Structure of the Egg
Most mammals ovulate an "egg" that has matured into a secondary oocyte; it is always the secondary oocyte that is fertilized. The secondary oocyte is produced along with the first polar body as a result of the first meiotic division. Both of these cells are encased in a thick glycoprotein shell called the zona pellucida. The image to the right shows a secondary oocyte from a mouse; residual follicle cells have been stripped away.
Genetically, the secondary oocyte that arrives in the oviduct is in metaphase of the second meiotic division. The metaphase plate is located inside the oocyte immediately below the first polar body.
The final structural feature of the egg that serves a critical
function during fertilization is a set of cortical granules. During
oogenesis, the oocyte develops thousands of small membrane-bound
granules that accumulate
in the
cortical cytoplasm, just beneath the plasma membrane.
The image to the right shows a mouse oocyte that has been stained to show cortical granules (all the small red dots). In this preparation, genomic DNA in the metaphase plate of the secondary oocyte (top right) appears bluish-white.
References
Text Reference:
Read the following reference pages in the McGraw-Hill text: 477 - 478
Visit the Following Website and review its contents.
Prerequisites
- The Process of Meiosis. ILO 4 adequately covers the topic. Click here to return to it.
