Collin College Dr. Chris Doumen BIOL 2402 Reproductive Systems 1 Reproductive System Most systems between males and females in the human body are similar in structure. The exception of course are the organs of the reproductive system. Memorable Quote from Kindergarten Cop (1990) Joseph: Boys have a penis, girls have a vagina! Detective John Kimble: Thanks for the tip. 2 1
Reproductive System The differences between males and females are genetically determined and are obviously apparent in the karyotype of an individual. Male Karyotype ( 2n=46) 22 pair of autosomal chromosomes 1 pair of sex chromosomes Males have an X and Y chromosome Females have 2 X chromosomes 3 Reproductive System Function of the differences is to perpetuate the species by means of gamete formation and reproduction Meiosis 4 2
Reproductive System: Meiosis Meiosis is the process of producing gametes (sperm and eggs) that have only 23 Chromosomes : One from each autosomal pair and one from the sex chromosomes. Homologous chromosomes are pairs of chromosomes with the same arrangement of genes and which we obtained from mother and 5 father. We thus have 23 pairs of homologous chromosomes. Reproductive System: Meiosis The separation of the chromosomes into the gametes is precise such that one from each pair of homologous chromosomes ends up in a gamete. In addition, crossing over re-arranges some of the genetic information such the unique new combination of genes occurs However, which one from each pair ends up in a gamete is complete random. This provides good mixing of maternal and paternal genes into 6 the next generation of gametes ( random segregation). 3
This picture represents a hypothetical case for a species with 6 chromosomes (3 pairs, 2n = 6). Blue and red colors indicate the parental origin of chromosomes. During metaphase one, crossing over occurs between sister chromatids and pairs of 7 homologous chromosomes are separated. Meiosis II is where the sister chromatids of each homologous chromosome set is split again into separate daughter cells. This final round thus produces cells with half the number of chromosomes. 8 4
Centriole pairs Interphase cell 2n = 4 Nuclear envelope Chromatin Interphase events As in mitosis, meiosis is preceded by DNA replication and other preparations for cell division. MEIOSIS I Crossover Spindle Sister chromatids Nuclear envelope fragments late in prophase I Centromere Prophase I Prophase events occur, as in mitosis. Additionally, synapsis occurs: Homologous chromosomes come together along their length to form tetrads. During synapsis, the arms of homologous chromatids wrap around each other, forming several crossovers. The nonsister chromatids trade segments at points of crossover. Crossover is followed through the diagrams below. Metaphase I The tetrads align randomly on the spindle equator in preparation for anaphase. Tetrad Dyad Chromosomes uncoil Nuclear envelopes re-form Cleavage furrow Anaphase I Unlike anaphase of mitosis, the centromeres do not separate during anaphase I of meiosis, so the sister chromatids (dyads) remain firmly attached. However, the homologous chromosomes do separate from each other and the dyads move toward opposite poles of the cell. Telophase I The nuclear membranes re-form around the chromosomal masses, the spindle breaks down, and the chromatin reappears as telophase and cytokinesis are completed. The 2 daughter cells (now haploid) enter a second interphase-like period, called interkinesis, before meiosis II occurs. There is no second replication of DNA before meiosis II. MEIOSIS II Prophase II Metaphase II Meiosis II begins with the products of meiosis I (2 haploid daughter cells) and undergoes a mitosis-like nuclear division process referred to as the equational division of meiosis. Anaphase II Telophase II and cytokinesis Products of meiosis: haploid daughter cells After progressing through the phases of meiosis and cytokinesis, the product is 4 haploid cells, each genetically different from the original mother cell. (During human spermatogenesis, the daughter cells remain interconnected by cytoplasmic extensions during the meiotic phases.) 10 5
Reproductive System: Meiosis 1 cell with 2n = 2 4 cells with n = 1 11 Main organs and structures are the testis epididymis Vas deferens Ejaculatory ducts Prostatic Urethra Membraneous Urethra Penile Urethra Penis Acessory organs are Seminal Vesicles Prostate Gland Bulbo-urethral glands 12 6
For a handy mnemonic, think of that refreshing soda, 7-Up: (S)emiferous tubules, (E)pididymis, (V)as deferens, (E) jaculatory duct, (N), (U)rethra, and (P)enis. 13 Major function of the testis is production of spermatozoans and testosterone. Testis consist out of many coiled tubules called seminiferous tubules. The cells that make up the walls of these tubules are the actual site of spermatozoan production. Spermatozoa move from the tubules into the rete testis, and then via efferent ductules into the epididymis, and into the vas deferens. The epididymis are the site where immature sperm cells undergo maturation. 14 7
Cross section through a seminiferous tubule 15 Seminiferous tubule (c) Interstitial cells Areolar connective tissue Myoid cells Spermatogenic cells in tubule epithelium Sperm 8
Spermatogenesis spermiogenisis Spermatogenesis is the testicular process in adult males that generates haploid gametes capable of fertilizing ova. The cells destined to undergo meiosis are called spermatogonia. 18 9
Spermatogenesis Spermatic cells give rise to sperm Mitosis Spermatogonia form spermatocytes Meiosis Spermatocytes form spermatids Spermiogenesis Spermatids become sperm 19 Mitosis of Spermatogonia Begins at puberty Spermatogonia Stem cells are in contact with the epithelial basal lamina They undergo mitotic division and produce a type A daughter cell and a type B daughter cell Type A cells maintain the germ cell line at the basal lamina Type B cells move toward the lumen and develop into primary spermatocytes 20 10
Mitosis of Spermatogonia Meiosis I Primary spermatocyte (2n) two secondary spermatocytes (n) Meiosis II Each secondary spermatocyte (n) two spermatids (n) Spermatid: small nonmotile cells close to the lumen of the tubule 21 Basal lamina Type A daughter cell remains at basal lamina as a stem cell Spermatogonium (stem cell) Type B daughter cell Tight junction between sustentacular cells Primary spermatocyte Cytoplasm of adjacent sustentacular cells Sustentacular cell nucleus Secondary spermatocytes Early spermatids Late spermatids Spermatozoa Cytoplasmic bridge Lumen of seminiferous tubule 11
Spermatogonium (stem cell) Mitosis Growth Enters meiosis I and moves to adluminal compartment Meiosis I completed Meiosis II Basal lamina Type A daughter cell remains at basal lamina as a stem cell Type B daughter cell Primary spermatocyte Secondary spermatocytes Early spermatids Late spermatids Spermatozoa 23 Spermiogenesis: Spermatids to Sperm Spermatids lose excess cytoplasm and form a tail, becoming spermatozoa (sperm) Major regions of mature sperm Head: genetic region; nucleus and helmetlike acrosome containing hydrolytic enzymes that enable the sperm to penetrate an egg Midpiece: metabolic region; mitochondria Tail: locomotor region; flagellum 24 12
Approximately 24 days Golgi apparatus Acrosomal vesicle Mitochondria Acrosome Nucleus 1 2 (a) Spermatid nucleus Centrioles 3 Microtubules Flagellum Excess cytoplasm Midpiece Head 4 5 Tail 6 7 25 Role of Sustentacular Cells Large supporting cells (Sertoli cells) Extend through the wall of the tubule and surround developing cells Provide nutrients and signals to dividing cells Dispose of excess cytoplasm sloughed off during spermiogenesis Secrete testicular fluid into lumen for transport of sperm 26 13
Tight junctions divide the wall into two compartments Basal compartment spermatogonia and primary spermatocytes Adluminal compartment meiotically active cells and the tubule lumen Tight junctions form a blood-testis barrier Prevents sperm antigens from escaping into the blood where they would activate the immune system Because sperm are not formed until puberty, they are absent during immune system development, and would not be recognized as self 27 Sertoli cells form tight junctions, preventing the body (blood) to form antibodies against these new strange cells. Under hot conditions, spermatogenesis is prone to errors, and overheated testes are associated with decreased fertility. 28 14
The tripartite chain regulates male reproduction via the Brain-Testicular Axis ( the hypothalamic-pituitary -gonadal (HPG) axis ) GnRH secretion from the hypothalamus drives LH & FSH secretion from the anterior pituitary gland. FSH develops the Sertoli cells and prompts them to produce and release Androgen Binding Protein (ABP) LH induces Leydig cells to produce testosterone. Leydig cells are the cells between the seminiferous tubules 29 In response to follicle stimulating hormone and testosterone, the Sertoli cells support and nourish developing spermatogenesis cells Sertoli cells phagocitize debris and secrete testicular fluid, easing the way for new spermatids Sertoli cells produce Androgen Binding Protein, (ABP) facilitating the effects of testosterone In response to luteinizing hormone, the Leydig cells produce steroids, including testosterone. Testosterone is needed for proper function of Sertoli cells and maturation of spermatids Sertoli cells secrete the hormone INHIBIN 30 15
Testosterone produced by Leydig cells diffuses into the testis area and ABP in Sustenticular (Sertoli) cells binds the testosterone, allowing it to exert its effect Regulation is by means of Negative feedback : Tetosterone inhibits GnRH and LH release Inhibin is produced when sperm production increases; it keep FSH levels in check as a result of negative feedback. 31 1 Anterior pituitary 8 2 GnRH Via portal blood 7 Inhibin 2 FSH 3 LH 4 Interstitial cells 6 Testosterone Sustentacular cell Spermatogenic cells 5 Somatic and psychological effects at other body sites Seminiferous tubule Stimulates Inhibits 32 16
33 Testicular androgens exert diverse effects in the reproductive tract, including the promotion of spermatogenesis. Testicular androgens are also responsible for: development of Wolffian ducts and early male characteristics developent of male brain transforms boys to men during puberty ( stimulates GH secretions and effect) In brain, some testosterone is converted into estrogen and determines the libido Testicular testosterone is converted in peripheral tissues to Dihydrotestosterone DHT via alpha-reductase. 34 17
35 Fluid secreted by Sertoli cells assist sperm transport along the lumen of the seminiferous tubules. The fluid also generates a pressure that moves the newly formed sperm into the Reti Testis. Eventually ends up in the head part of the epididymis During passage through epididymis sperm will undergo final maturation process which takes about 14 days; so sperm entering initial part of epididymis is incapable of motility and capacity to fertilize. Epididymis also serves as a storage place for sperm up to a month. Older sperm are degraded here and reabsorbed into the epithelium through phagocytosis 36 18
Accessory Glands Seminal Vesicles : paired pouches located posteriorly at the base of the bladder secretes an alkaline fluid containing fructose, prostaglandins and fibrinogen helps to neutralize the acid environment in the female reproductive tract (sperm is killed by acidic media) fructose provides energy for the mitochondria, fibrinogen makes semen stick and adhere inside the vagina and prostaglandin creates small contractions inside the to propel the sperm along its journey secretions provide about 60% of total seminal fluid 37 Prostate Gland : a single spherical gland located at base of bladder and encircles the urethra produces a slightly acidic fluid containing citric acid and other sperm activation enzymes such as fibrinolysin makes up 25 % of seminal fluid Bulbo-urethral Gland : paired glands below the prostate gland produce a alkaline fluid as well that neutralizes the acidity of the male urethra Sperm count is around 50-150 million per ml. Sperm count below 50 million/ml reduces fertility chances and if count is lower than 20 million per ml, a male is considered clinically infertile 38 19
Causes of abnormal Sperm Thermal exposure Heat potentially damages spermatogenesis or the sperm that is stored in the epididymis. Testicular surgery Infections Radiation Substance abuse / Medications Trauma 39 Penis Consist out of 3 cylindrical compartments running length-wise 2 dorsal corpora cavernosa 1 ventral corpus spongiosum During sexual stimulation, small arteries feeding these compartments relax and blood floods the compartments causing erection. Erection is under Para-sympathetic control vasodilation is further influenced by nitric oxide (NO) NO produces cgmp which in turn causes smooth muscle relaxation Ejaculation is under sympathetic control 40 20
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Corpus cavernosum Nitric Oxide released during sexual stimulation Blood vessel smooth muscle layer Activates Guanyl Cyclase Viagra (inhibits PDE) Production of cyclic GMP Smooth muscle relaxation Phosphodiesterase (PDE) (Breaks down cgmp) Increased blood flow Erection 22