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Lecture 09

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University of Toronto Scarborough
Biological Sciences
Ian Brown

LECTURE 9 Spermatogenesis: The development of the male sex cells. Why is spermatogenesis a good model system? Sperm cells are highly specialized, they can move around on their own and no other cells in the body can undergo independent motility. It occurs in the testes and in the seminiferous tubules. The central cavity is where the mature sperm cells are found. At the edge of the cell, there are stem cells (spermatogonia) they produce more stem cells, they stop dividing and begin to differentiate into sperm cells. As we go inward we see more and more mature sperm cells until we reach the center which holds the complete adult mature sperm cell. Marker protein, protamine. In some species sperm development is seasonal. It occurs during a certain season of the year, like fish and amphibians. In humans and mammals it is going on all the time. (diagram -Cross section of the testes) The testes are covered by protective layer (tunica). Inside, the different segments are divided by septa, contained in this are the seminiferous tubules. Each tubule has a lumen where they are collected into a common area (epididymis) and where the mature sperm cells exit. (diagram) Circular structures, each one is a seminiferous tubule. Inside each structure shows stem cells around the outside. A hormone will act on some of the stem cells pushing them into differentiation. The mature sperm are located in the centre. Function of sperm: Three (3) functions; Highly differentiated and specialized. 1) Prime function of the sperm cell is the transfer of genetic information from male to female in the process of fertilization. To do this, the sperm cells needs to have independent motility (it must move by itself) to reach the egg. This is the only cell in the body that can do this. 2) It also protects the male genetic information when moving from the male and into the egg. Sometimes this is internal in the female, in other species fertilization is external (amphibians) where female lays the eggs first and male fertilizes after. 3) There is a change in environment and the genetic information needs to be protected when moving from one animal to the next. Mature sperm: Can be divided into three (3) parts; • Head • Mid-piece • Tail What are in the three (3) parts? 1 Head; The head of the sperm contains the male nucleus and the male genetic information. This needs to be protected. The DNA in the sperm head is highly packed. This helps to streamline the sperm. At the tip of the sperm head is the acrsome. This contains a zymogen that breaks a pathway through the female jelly coat to allow the sperm through. Manchette, is a parallel array of microtubules. This gives structure to the sperm head. It also has very little cytoplasm as this has been shed. The sperm head is totally shut down as far as RNA synthesis is concerned. There is no RNA capacity in the sperm head. Midpiece; The midpiece contains a concentration of mitochondria for energy. This is the battery of the sperm. Sperm has a limited half-life, the midpiece battery has a limited lifespan, therefore the sperm then has a limited lifespan. This is the energy supplier for motility of the sperm. Tail; Flagellum. The tail is formed of microtubules. There is a ring of nine (9) microtubules around the outside and two (2) in the middle, this is the classic microtubule structure of the tail. Contraction of the nine (9) and two (2) central causes the motility of the tail. The differential sliding of the tubules results in the movement of the tail. Variety of sperm morphology: There is a great deal of variation of sperm cells in different species. This is in the shape of the sperm head and the length of the tail. They will vary depending on the species on animal. In general, sperm are smaller and simpler when fertilization is external. Specializations may be adaptations between species. (examples provided on slide) Rat vs. Human Rat has hook like head with tail. Human has round (egg shaped) head with very thin tail. (diagram) Two (2) Phases of Spermatogenesis: Divided into two (2) distinct phases; Meiosis Spermiogenesis Meiosis; This is a reduction/division stage. Most cells are called somatic cells and have a diploid compliment of DNA. However, the sex cell are haploid. Therefore, it undergoes Meiosis. Two (2) haploid cells (male and female) join to make a diploid cell to complete the DNA compliment. Diploid cells make more diploid cells by meiosis, which is reduction/division, diploid cells are reduced to the haploid state. Spermiogenesis is the final stage, after meiosis is complete. 2 At the outer edge of the seminiferous tubule contains the spermatogonium, the center of the tubule contains the final stage of adult sperm. Spermatogonium are the stem cells (2n) means the cell is diploid. It is undergoing regular mitosis. These cells are piling up within the tubule. Testosterone impacts some of the stem cells and stops the normal mitotic division and pushes these cells to begin meiosis and begin the reduction/division process. It doubles the DNA to produce the Primary Spermatocyte (4n). After this, there is the first miotic division which forms two (2) secondary spermatocyte (2n). Then there is the second mitotic division which is the spermatid (1n) forming four (4) haploid spermatids. In all of the stages, there has been no structure changes. Spermiogenesis; All the changes have happened to produce the spermatid. Meiosis completes before Spermiogenesis begins. Hormones and spermatogenesis; Testosterone; Spermatogenesis is under hormonal control. This process is from the trout (fish) in a lab. Spermatogenesis can be triggered at any time when they are injected with the correct hormone in a lab setting. In nature, There is a seasonal change and this is detected by the male sensory system in the hypothalamus to produce gonaldotropin-releasing hormone, hormone affects pituitary gland and triggers the production of two hormones (lutenizing hormone [LH]) which is released into blood stream and effects the cells in the testes (Leydig) they then make the hormone called Testosterone that act to help to create spermatogonia. Pituitary gland is also making another hormone (follicle stimulating hormone [FSH]) that affects sertoli cells in the testes, these are help cells that support the production of spermatogenesis. Sertoli cells make a hormone (androgen binding protein) this hormone retains testosterone in the tubule to trigger spermatogenesis. Therefore, they need testosterone at a certain level before spermatogenesis begins. They need this binding protein. (diagram) Outer edge is at the bottom, this is where the stem cells are along the out edge. As we travel (up) this is the center of the lumen and the mature sperm can be seen. Two (2) hormones (LH and FSH) are the two (2) gonadotrophins that are made in the pituitary gland. The sertoli cells are close to the spermatogonia cells they give support and control the rate of production of the mature sperm. Sertoli cells also provide a blood-testes barrier by forming tight junctions with other sertoli cells. This provides a barrier from things entering from the blood into the tubules. Sertoli also produces transferrin, this is a protein that requires iron to undergo spermatogenesis. This helps to transport that iron into the sperm cells. Sertoli also produces inhibin, this is secreted into the blood stream and prevents FSH production by controlling the level of FSH in a classic feedback mechanism. 3 Spermatogenesis is divided into two (2) phases; In the first phase there are no morphological changes. Spermiogenesis is when the sperm changes in shape from a round cell like to a mature sperm. Spermiogenesis can be divided into five (5) phases; 1. Acrosomal granule 2. flagellum growth 3. loss of cytoplasm 4. elongation of sperm nucleus 5. formation of midpiece There is no particular order to these phases. (diagram showing all the stages) Acrosomal granule; Begins in the spermatid in the golgi apparatus. Golgi breaks off acrosomal granules, these granules form the vesicle that attaches to one end of the nuclear envelope. This envelope is a membrane. Acrosomal vesicle fuses with outer membrane and collapses in on the outer membrane. Within the vesicle is the granules and places these granules at the tip (head) of the sperm. The head contains a zymogen enzyme (inactive) until the sperm reaches the egg. When conditions are right the zymogen will be activated and will make a pathway thr
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