Embryology 3. Spermatogenesis:

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Rodney Leonard
6 years ago
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1 Embryology 3 Spermatogenesis: The 2 testis in males are each divided into lobes and lobules by connective tissue septa forming 250 lobule and in each lobule there are 1 to 4 seminefrous tubule ( so almost 1000 seminefrous tubules are found in each testis) In the basement membrane of Seminefrous tubule we can find primordial germ cells (arises from wall of young sac in the 4 th week and reach in 5 th week) these cells would give spermatogonia that is used to form sperms Between primordial cells, leydig cells are found and are used to secrete testosterone under the effect of LH (luteinizing hormone) and gonadotrophic hormone that are produced from anterior lobe of pituitary gland at puberty age. Testosterone stimulates the spermatogonia to divide. Supporting cells, which are derived from the surface epithelium of the gland in the same manner as follicular cells (in ovaries), become sustentacular cells, or Sertoli cells or supporting cells. Sertoli cells support and protect the germ cells, participate in their nutrition, and assist in the release of mature spermatozoa, plus their role in phagocytosis. These 3 cells (primordial germ cells, sertoli cells and leydig cells) are all found in the seminefrous tubule. Spermatogenesis: includes all of the events by which spermatogonia are transformed into spermatozoa (sperms). Spermatogenesis begins at puberty age under the effect of LH and FSH (follicle stimulating hormone).

2 Spermatogenesis is regulated by luteinizing hormone (LH) that is produced by the anterior lobe of pituitary gland. LH binds to receptors on Leydig cells and stimulates testosterone production, which in turn binds to Sertoli cells to promote spermatogenesis. Follicle stimulating hormone (FSH) is also essential because its binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins. At birth, germ cells in the male can be recognized in the sex cords of the testis as large, pale cells surrounded by supporting (sertoli) cells that stay attached to spermatid until they are released in form of sperms. Spermatogonia will give primary spermatocytes that will divide in return to give secondary spermatocytes that will develop into spermatids. spermatids will stay in touch with sertoli cells for a certain period of time and then it will develop to form spermatozoa (sperms). If we took a look at the male organ system we'll find: testis, ducts (vas deferens, urethra, ejaculatory), and glands which give secretions and these secretions together with sperms form a fluid called semen. At regular intervals, cells emerge from this stem cell (spermatogonia) population to form dark spermatogonia that turns into pale spermatogonia which contains type A and B and all these cells are needed for mitosis to maintain the spermatogonia (mother cell). Only type B cell enters spermatogenesis and gives primary spermatogonia ( has 46 chromosomes ) that will undergo meiosis 1 and 2. Primary spermatocytes then enter a prolonged prophase (22 days) during meiosis 1 and are followed by rapid completion of meiosis I

3 and formation of secondary spermatocytes (having 23 chromosomes.) During the second meiotic division, these cells immediately begin to form haploid spermatids (not yet called sperm cells because its still attached to sertoli cell that gives it protection and nutrition.) Throughout this series of events, from the time type A cells leave the stem cell population to formation of spermatids, cytokinesis is incomplete, so that successive cell generations are joined by cytoplasmic bridges.(means spermatids remain attached to sertoli cell by cytoplasm..when It is released, it leaves residual cytoplasm behinds it) connection between sertoli cells forms alduminal zone (occluded zone or junctional complex) in which only spermatogonia is found (between basement membrane and occluded zone) and after this zone towards the lumen we have primary and secondary spermatocytes This zone is very imp for protection of primary spermatocytes and spermatids when they enter division..there is no protection for spermatogonia because they are found in large no. and all undergo mitosis. Check the diagram in slide that has the summary of type A and type B cells. (V.IMP) If we took 2 type B cells; they will divid to form 4 primary cells then 8 secondary cells and in the end 16 spermatids ( half with X and half with Y chromosome) that will later develop into sperms.

4 Spermiogenesis Its the series of changes resulting in the transformation of spermatids into spermatozoa is spermiogenesis. These changes include (a) formation of the acrosome, which covers half of the nuclear surface and contains lytic enzymes to assist in penetration of the egg and its surrounding layers during fertilization(golgi is formed first then acrosomal stage) (b) condensation of the nucleus (contains half no. of chromosomes) (c) formation of neck, middle piece (contains large no. of mitochondria for production of energy used in motility of tail), and tail (d) shedding of most of the cytoplasm (maturation stage in which spermatozoa is released). In humans, the time required for a spermatogonium to develop into a mature spermatozoon is approximately 64 days. But this process is ongoing and that is why we always find large no. of sperms. Note: from sperms found in semen, only one can reach the mature ovum for fertilisaton in what is called "long journey". When fully formed, spermatozoa enter the lumen of seminefrous tubules. They are pushed toward the epididymis by contractile elements in the wall of the seminefrous tubules Spermatozoa obtain full motility in the epididymis. Maturation phase in the epididyms take days Note: motility of sperm is different than viability. Motility is the ability from sperm to swim to ovum where as viability is the healthiness of the sperm (less abnormalities and more normal sperms).

5 The testicular fluid in which the spermatozoa are transported is produced by Sertoli cells and rete testis (it s found in the hylum of the testis and it is the point of connection of seminefrous tubules to a duct used to carry sperm to epididymis.) This fluid contains: Steroids, proteins, ions, Androgen binding protein (ABP), testosterone Life expectancy of ejaculated sperms are hours Factors affecting spermatogenesis 1. Temperature (that s why testis are found in scrotum outside male body 2 to 3 degrees below body temperature..which the the optimum temperature for formation of sperms. ) Note: one of the most common diseases related to temprature is the varicocele of testis (which is the torsos of plexus of veins and engorgement of blood and this results in killing of sperms ) and takes place on left testis more because The spermatic vein on the left side is longer and takes a longer route back toward the heart. It connects to the left kidney vein, which has higher blood pressure within it. The higher blood pressure on this side increases the likelihood of swelling in a weakened vein. By contrast, the right testicular vein is usually shorter. It connects directly to the vena cava - the primary vein leading to the heart - which is characterized by lower blood pressure. For this reason, varicoceles of the right testicle are uncommon. 2. vitamin E ( V.imp in spermatogenesis) 3. Gonadotrophic hormones : FSH and LH 4. Toxins (asbestos), X-ray exposure, vitamin deficiencies, injuries

6 Note: Azospermia means no sperm in semen.legospermia means low count of sperms in semen. Differentiation of early spermatid to sperm is called cytodifferentiation. Oogenesis Ovary is divided into cortex ( that have germ cells) and medulla (that is supplied by blood vessels ) Primordial follicle is an oocyte surrounded by flat layer of epithelial cells. Every month, 10 to 20 primordial follicles grow into primary follicle in which flat cells increase in no. by mitosis and turn into cuboidal cells. Note: here mitosis takes place in flat cells and not oocyte! Primary cells are then converted into secondary in which no. of cells surrounding the oocyte increase even more forming granulosa cells and a fluid-filled space called follicular space or equar space starts to form.. From the 20 primordial cells only one grow to form graafian follicle ( mature ovum) and the rest get disintegrated. The graafian follicle is the one that will get ovulated in presence of sperms or will get disintegrated and menstrual cycle will take place. The release of the mature ovum from the surface of ovary is called ovulation and takes place on day 14 from the start of menstruation. It will then go to the ampulla of fallopian tube for 48 hours. Maturation of Oocytes Begins Before Birth Once primordial germ cells have arrived in the gonad of a genetic female, they differentiate into oogonia (by mitosis).

7 These cells undergo a number of mitotic divisions and, by the end of the third month, are arranged in clusters surrounded by a layer of flat epithelial cells All of the oogonia in one cluster are probably derived from a single cell. the flat epithelial cells, known as follicular cells, originate from surface epithelium covering the ovary. During the next few months, oogonia increase rapidly in number, and by the fifth month of prenatal development, the total number of germ cells in the ovary reaches its maximum, estimated at 7 million At this time, cell death begins, and many oogonia as well as primary oocytes become atretic (degenerated). By the seventh month, the majority of oogonia have degenerated except for a few near the surface After birth the number is to oocytes. All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat epithelial cells but instead of proceeding into metaphase, they enter the diplotene stage, a resting stage during prophase that is characterized by a lacy network of chromatin. (this stage remains until menopause..this is because every month only 10 to 20 primary oocyte grow but the rest remain dormant.) Primary oocytes remain in prophase and do not finish their first meiotic division before puberty is reached, because of oocyte maturation inhibitor (OMI), a substance secreted by follicular cells A primary oocyte, together with its surrounding flat epithelial cells, is known as a primordial follicle. Maturation of Oocytes Continues at Puberty.

8 During childhood most oocytes become atretic; only approximately 400,000 are present by the beginning of puberty, and fewer than 500 will be ovulated. Note : Dormant stage of primary oocyte may be affected by time and cause abnormalities in the chromosome and that s why the risk of abnormal fetus increases after the age of 35 in woman. 20 follicles selected from this pool begin to mature, passing through three stages: 1) primary or preantral; 2) secondary or antral (also called vesicular or Graafian); 3) preovulatory (before ovulation) The antral stage is the longest, whereas the preovulatory stage encompasses approximately 37 hours before ovulation. As the primary oocyte begins to grow, surrounding follicular cells change from flat to cuboidal and proliferate to produce a stratified epithelium of granulosa cells, and the unit is called a primary follicle (primordial follicle). Granulosa cells rest on a basement membrane separating them from surrounding stromal cells that form the theca folliculi. While the primary follicle is growing a layer of glycoproteins called zona pellucida forms (secreted by the granulosa cells and oocyte) and surrounds the oocyte for protection. This layer disintegrates only after the 5 th day of fertilization when it moves to the uterus and before it gets implanted! As follicles continue to grow, cells of the theca folliculi organize into an inner layer of secretory cells, the theca interna ( composed of cells having characteristics of steroid secretion and is rich in blood vessels) and an outer fibrous capsule, the theca externa, which gradually merges with the ovarian stroma

9 As development continues, fluid-filled spaces appear between granulosa cells.coalescence of these spaces forms the antrum, and the follicle is termed a secondary or vesicular or Graafian follicle. Note: Initially, the antrum is crescent shaped, but with time, it enlarges Granulosa cells surrounding the oocyte remain intact and form the cumulus oophorus near one pole. Also, small, finger-like processes of the follicular cells extend across the zona pellucida and interdigitate with microvilli of the plasma membrane of the oocyte.these processes are important for transport of materials from follicular cells to the oocyte At maturity, the secondary follicle may be 25 mm or more in diameter When the secondary follicle is mature, a surge in luteinizing hormone (LH) induces the preovulatory growth phase. Meiosis I is completed, resulting in formation of two daughter cells of unequal size, each with 23 double structured chromosomes One cell, the secondary oocyte, receives most of the cytoplasm the other, the first polar body, receives practically none. The doctor stopped here and said he will continue next time.. but as a conclusion..in the end 1 mature ovum will form and 3 polar bodies will get disintegrated

Cell Divisions. The autosomes represent the whole body. * Male Sex Chromosomes: XY * Female Sex Chromosomes: XX

Cell Divisions. The autosomes represent the whole body. * Male Sex Chromosomes: XY * Female Sex Chromosomes: XX Cell Divisions Each Cell (including gonads) has 46 chromosomes (23 pairs of chromosomes: 22 pairs of autosomes, 1 pair of sex chromosomes) which are located in the nucleus). The autosomes represent the