Gametogenesis PDF

INTRODUCTION TO BODY STRUCTURE 85 GENERAL EMBRYOLOGY INTRODUCTION TO BODY STRUCTURE 86 Meiotic divisions and crossing over Spermatogenesis Pinterest /...

INTRODUCTION TO BODY STRUCTURE 85 GENERAL EMBRYOLOGY INTRODUCTION TO BODY STRUCTURE 86 Meiotic divisions and crossing over Spermatogenesis Pinterest / Career power INTRODUCTION TO BODY STRUCTURE 87 GAMETOGENESIS Definitions Primary sexual organ: it is the organ containing the cells capable of forming a gamete (half of the future human being). They are the testis in males and the ovaries in females. Primary sex cells: The cells which are capable of forming a gamete. They are the spermatogonium in males and oogonium in females. Gametogenesis: it is formation of a gamete (sperms in males and ova in females). Mitotic division: is the division of a cell leading to two daughter cells similar to each other and to the mother cell. Meiotic division: is the division of a cell leading to two daughter cells not similar to each other or the mother cell. It shows a decrease in the number or the amount of chromosomes and DNA. In gametogenesis there are two meiotic divisions: First meiotic division: The 46 chromosomes in the nucleus of a cell arranges into two columns (each contains 23 chromosomes). Each chromosome incompletely splits into two chromatids. Crossing over: each chromosome exchanges a segment with its neighboring chromosome (this leads to uncounted number of chromosomal variations). The cell divides into 2 cells (each containing 23 chromosomes). Second Meiotic division: Each of the 23 chromosomes splits completely into two chromatids. Each chromatid develops into complete chromosome (note that the two chromatids are not identical, So, the newly formed chromosomes are not similar). The cell divides into 2 cells (each containing 23 chromosomes). SPERMATOGENESIS Definition: formation of a sperm from spermatogonium. Site: in seminiferous tubules of the testis. Process: Spermatogonia lies on the basement membrane of the seminiferous tubules. It contains 46 chromosomes (44+X+Y). Each spermatogonium divides by mitosis forming 2 daughter spermatogonia (each contains 46 chromosomes (44+X+Y). One of the daughter spermatogonia (spermatogonia A) remains in contact with basement membrane while the other (spermatogonia B) starts to mature. Spermatogonia B enlarges in size and transforms into primary spermatocyte (44+X+Y chromosomes). Primary spermatocyte is the largest germ cell. Primary spermatocyte divides by 1st meiotic division forming two secondary spermatocytes (22+X and 22+Y chromosomes). This division decreases the number of chromosomes. Each secondary spermatocyte divides by 2nd meiotic division forming two spermatids (22+X or 22+Y). This division decreases the amount of DNA in each chromosome. INTRODUCTION TO BODY STRUCTURE 88 Sperm Scanning EM of sperms Abnormal forms of sperms Vector stock / Quara / ResearchGate INTRODUCTION TO BODY STRUCTURE 89 Spermiogenesis: is the transformation of spermatid into sperm: The nucleus of the spermatid becomes the head of the sperm. The Golgi apparatus of the spermatid becomes the head (acrosome) cap of the sperm (containing enzymes to facilitate the penetration of ova). One of the centrioles of the spermatid elongates forming the axial filament of the sperm (middle piece and tail). The mitochondria of the spermatid surrounds the upper part of the axial filament forming the middle piece. The rest of the axial filament forms the tail of the sperm. The neck is a narrow junction between the head and the middle piece. The sperm is about 55 μm (0.055 mm) long where the head is 5 μm and the tail is 50 μm (1/10). Characteristics of spermatogenesis: The process from the mitotic division of spermatogonia to the formation of the sperm lasts for about 2 months. The spermatogenesis results in formation of 4 sperms from each spermatogonium with a preservation of a copy for further spermatogenesis (that’s why spermatogenesis does not stop by age). The sperms passing from testis are immotile, they acquire motility during passage in epididymis (2 weeks). Each testis is divided into 200 compartments, each contains 2 seminiferous tubules, each of them is 2 feet long. A large number of sperms are continuously formed. SEMINAL FLUID (SEMEN) ❖ It consists of sperms (formed in testis and transported through epididymis, vas deference and ejaculatory duct) and secretions from male genital glands (mainly seminal vesicles and prostate). Normal parameters of semen: Amount: 2-6 ml/ ejaculate. Lower volume is called hypospermia (hypo = below), the absence of semen with ejaculation is known as aspermia (a = no). Sperms count: 20-200 million sperm/ ml (40-1200 million sperms/ ejaculate). Lower values are called oligozoospermia (oligo = few). Vital sperms: ≥ 60%. Lower values are called necrozoospermia (necro = dead). Normal sperm forms: (≥ 4%), Some abnormal forms are capable of fertilization. Lower values are called teratozoospermia (terato = malformed). Sperm motility: ≥ 40% and ≥ 30% with aggressive motility. Lower values are called asthenozoospermia (astheno = weak). ❖ Out of the ejaculated sperms, only 500 sperms reach the ampulla of Fallopian tubes (site of fertilization). This process lasts 2 hours after ejaculation. ❖ Sperms survives for 2 days in female genital tract. INTRODUCTION TO BODY STRUCTURE 90 Oogenesis Pinterest INTRODUCTION TO BODY STRUCTURE 91 OOGENESIS Definition: formation of a ova from oogonium. Site: in the cortex of ovaries. Process: Oogonia contains 46 chromosomes (44 + X + X). Oogonium enlarges in size and transform into primary spermatocyte (44+X+X chromosomes). Primary oocyte divides by 1st meiotic division forming two cells; one secondary oocytes (22+X) and the other degenerates forming 1st polar body (22+X). Secondary oocyte divides by 2nd meiotic division forming two cells; mature ova (22+X) and the other degenerate forming 2nd polar body (22+X). Characteristics of Oogenesis: Oogenesis timing: ⎯ During female intrauterine life, all oogonia are transformed to primary oocytes, which starts 1st meiotic division but does not complete it. ⎯ With each ovarian cycle, primary oocyte completes the first meiotic division forming secondary oocyte and 1st polar body, the secondary oocyte starts the 2nd meiotic division but does not complete it, this is the cell which is ovulated. ⎯ If fertilization occurs, secondary oocyte completes the 2nd meiotic division forming mature ova and 2nd polar body. Oogenesis population ⎯ During intrauterine life, the oogonia undergoes several mitotic divisions reaching 5 million, most of them degenerate and only 1 million are transformed into primary oocytes (at birth), only 40.000 primary oocytes survive to the age of puberty. ⎯ Starting from puberty, each ovarian cycle 20-100 primary oocytes start the process of ova formation, only one of them succeeds. The large consumed number are necessary for hormonal production to complete the process. ⎯ The oogenesis results in formation of 1 ovum from 20-100 oocyte (that’s why oogenesis stops by age leading to menopause). ⎯ Only one ovum is produced each month. It lasts for 12-24 hours only. INTRODUCTION TO BODY STRUCTURE 92 Follicle development Pinterest INTRODUCTION TO BODY STRUCTURE 93 OVARIAN FOLLICLES AND OVULATION Primary follicle: during intrauterine development, each primary oocytes is surrounded by a single layer of flat follicular cells. Secondary follicle: at puberty, and parallel to the development of primary oocyte into secondary oocyte, the follicular cells become granulosa cells (multilayers of cubical cells containing granules (hence the name). Tertiary follicle: cavities start to appear between granulosa cells. Mature Graafian follicle: The cavities enlarge and unite forming an antrum (= chamber). The outer layer of the follicle is called stratum granulosum (= granular layer). The secondary oocyte with its surrounding cells is called cumulus oophorous (= ovarian elevation) and is formed of: ⎯ Secondary oocyte. ⎯ Zona pellucida (= transparent zone): a membrane surrounding the secondary oocyte. ⎯ Corona radiata (= radiating crown): the granulosa cells around the zona pellucida. The ovarian tissue due to the compression of enlarging follicle becomes compact and form a capsule called theca folliculi which is further divided into: ⎯ Theca interna: formed of connective tissue cells which secrete estrogen. ⎯ Theca externa: formed of fibrous tissue. Ovulation: due to the enlargement of the follicle and pressure on the ovarian cortex, the overlying cortex becomes ischemic and rupture leading to: The secondary oocyte with the surrounding zona pellucida and corona radiata leaves the ovary and enters the Fallopian tube. The rest of the granulosa cells and surrounding theca interna cells start to accumulate yellow pigments forming corpus luteum (= yellow body) and secrete progesterone and small amount of estrogen. ⎯ If fertilization occurs, the dividing cells secrete HCG which stimulates the corpus luteum to enlarge and continue secreting progesterone during the first half of pregnancy (corpus luteum of pregnancy) till it is replaced by placenta. ⎯ If no fertilization occurs the corpus luteum gradually degenerates within ten days (corpus luteum of menstruation → corpus albicans). INTRODUCTION TO BODY STRUCTURE 94 Ovarian cycle Pinterest INTRODUCTION TO BODY STRUCTURE 95 OVARIAN CYCLE ❖ It is cyclic changes which occur in the ovarian follicles under the effects of the pituitary hormones. ❖ This cycle occurs monthly from puberty to menopause. Phases of ovarian cycle: Follicular phase: the pituitary gland starts to secrete FSH which leads to: Primary oocyte completes the 1st meiotic division forming secondary oocyte and 1st polar body. Maturation of primary follicle to mature Graafian follicle. The granulosa cells secrete estrogen which at certain level has negative feedback on FSH (decreases its secretion) and stimulates LH secretion (also from pituitary gland). The high estrogen level of this phase is responsible for the proliferative phase of uterine cycle. Ovulation: caused by LH surge leading to liberation of secondary oocyte with its surroundings from the ovary. Luteal phase: LH secreted from pituitary gland causes corpus luteum to secrete progesterone and small amount of estrogen (both suppress FSH stopping crossing cycles). If fertilization occurs, the corpus luteum enlarges and continues secreting progesterone leading to inhibition of FSH → stopping new cycles. If fertilization does not occur, corpus luteum gradually degenerates, and progesterone level decreases leading to an increase in FSH hormone causing initiation of a new cycle. The high progesterone level of this phase is responsible for the secretory phase of uterine cycle. INTRODUCTION TO BODY STRUCTURE 96 Ovarian and uterine cycles Pinterest INTRODUCTION TO BODY STRUCTURE 97 UTERINE (MENSTRUAL) CYCLE ❖ The uterus is a muscular organ lined with endometrium formed of basal, compact and spongy layers. ❖ The uterine cycle is a monthly changes which occurs in the uterine endometrium under the effect of the ovarian hormones. ❖ This cycle occurs monthly (21-35 days) from puberty to menopause. Phases of Uterine cycle: Proliferative (estrogenic – follicular) phase : Under the effect of estrogen (secreted by the granulosa cells during the follicular phase of ovarian cycle). The estrogen stimulates the basal layer to develop leading to construction of compact and spongy layers (the cells increase in number, the glands and the arteries increase in number and size). It lasts for 10 days. Secretory (progestational – luteal) phase : The endometrium proliferates under the effects of progesterone (mainly) and estrogen secreted by corpus luteum in the luteal phase of the ovarian cycle. The cells enlarge and accumulate nutrients, the glands dilate and are filled with glycogen and the arteries dilate and become spiral. It lasts for 14 days. If fertilization occurs, corpus luteum of pregnancy continues to secrete progesterone keeping the enlarged endometrium with the dividing cells implanted into it (the endometrium will be called decidua). If fertilization does not occur, the corpus luteum degenerates with decreased progesterone level leading to vasoconstriction of the uterine arteries causing ischemia followed by shedding and bleeding. Menstrual phase: It occurs due to decreased estrogen and progesterone levels due to degeneration of corpus luteum. Shedding of the compact and spongy layers of endometrium occurs leaving only the basal layer. It lasts for 3-7 days. MENOPAUSE ❖ It is cessation of ovarian cycle due to exhaustion of primary follicles during previous cycles → little number of follicular cells is present to be stimulated by FSH → decreased estrogen and progesterone levels → cessation of uterine cycle. INTRODUCTION TO BODY STRUCTURE 98 Fertilization Cleavage, morula and blastocyst Quora / Shutter stock