Spermatogenesis and Oogenesis: Last Lecture 2024 PDF

Spermatogenesis In the Beginning Males start producing sperm when they reach puberty, which is usually from 10-16 years old. Biological males continually produce sperm in large quantities (~200 million a day). This maximizes the likelihood of sperm reaching the egg following ejaculation. Sperm prod...

Spermatogenesis In the Beginning Males start producing sperm when they reach puberty, which is usually from 10-16 years old. Biological males continually produce sperm in large quantities (~200 million a day). This maximizes the likelihood of sperm reaching the egg following ejaculation. Sperm production occurs in the testes of the male, specifically in the seminiferous tubules. In the testicles, a blood testis barrier forms to keep the tubules separate from the systemic circulation. Protecting the Sperm Sertoli cells form the blood testis barrier. This is important in preventing substances found in blood from affecting the developing sperm. These products might include hormones or waste products. It is also important as it prevents the immune system of the male from recognizing the sperm as foreign – the sperm are genetically different from the male and will express different surface antigens. Forming Functional Sperm Spermatogonia are the initial pool of diploid cells that divide by mitosis to give two identical cells. One of these cells will be used to replenish the pool of spermatogonia – these cells are A1 spermatogonia. This replenishment of spermatogonia means that males are fertile throughout their adult life. The other cell – type B spermatogonium – will eventually form mature sperm. Type B spermatogonia replicate by mitosis several times to form identical diploid cells linked by cytoplasm bridges, these cells are now known as primary spermatocytes. Primary spermatocytes then undergo meiosis: Meiosis I produces two haploid cells, known as secondary spermatocytes. Meiosis II produces four haploid cells, known as spermatids Maturation The cytoplasmic bridges break down and the spermatids are released into the lumen of the seminiferous tubule – a process called spermiation. The spermatids undergo spermiogenesis (remodeling and differentiation into mature spermatozoa) as they travel along the seminiferous tubules until they reach the epididymis. From the seminiferous tubule, cells will travel to the rete testis. This acts to “concentrate” the sperm by removing excess fluid. Then, cells move to the epididymis where the sperm is stored and undergoes the final stages of maturation. Spermatogenesis takes approximately 70 days, therefore for sperm production to be continuous and not intermittent, multiple spermatogenic processes are occurring simultaneously within the same seminiferous tubule, with new groups of spermatogonia arising every 16 days (spermatogenic cycle). Each of these populations of spermatogenic cells will be at different stages of spermatogenesis. Following Ejaculation Note that once sperm leave the male body and enter the female reproductive tract, the conditions cause the sperm to undergo capacitation. This is the removal of cholesterol and glycoproteins from the head of the sperm cell to allow it to bind to the zona pellucida of the egg cell. Oogenesis Oogenesis differs from spermatogenesis in that it begins in the foetus before birth. Primordial germ cells (which originate in the yolk sac of the embryo) move to colonize the cortex of the primordial gonad. Replication by mitosis peaks at approximately 7 million by mid-gestation (~20 weeks). Cell death occurs after this peak leaving roughly 2 million cells. Meiosis I begins before birth and forms primary oocytes. There is therefore a finite supply of ova. Primary oocytes are arranged in the gonads as clusters. They have flattened epithelial cells surrounding them, and this is called the primary follicle. During childhood, further atresia (cell death) occurs, leaving ~40,000 eggs at puberty. Once puberty begins, several primary oocytes (15-20) begin to mature each month, although only one of these reaches full maturation to become an oocyte. The primary oocytes undergo 3 stages: Pre-antral Antral  Pre-Antral Stage The primary oocyte is still in meiosis I but will grow dramatically in this stage. The follicular cells grow and proliferate to form a stratified cuboidal epithelium. Now, we call these granulosa cells, and they secrete glycoproteins. These chemicals form the zona pellucida around the primary oocyte. Surrounding connective tissue cells also differentiate to become the theca folliculi, a specialised layer of surrounding cells that is responsive to LH and can secrete androgens under its influence. Antral Stage Fluid-filled spaces form between granulosa cells, these eventually combine to form a central fluid-filled space called the antrum. We now call the follicles secondary follicles. In each monthly cycle, one of these secondary follicles becomes dominant and develops further under the influence of FSH, LH, and oestrogen. Pre-Ovulatory Stage The LH surge induces this stage and meiosis I is now complete. Inside the follicle, 2 unequally sized haploid cells form. One of the daughter cells receives far less cytoplasm than the other and forms the first polar body, which will not go on to form an ovum. Another haploid cell is also formed, known as the secondary oocyte. Both daughter cells then undergo meiosis II. An initial polar body will replicate to give two polar bodies but the secondary oocyte arrests in metaphase of meiosis II. This happens 3 hours prior to ovulation Ovulation Now, the follicle has grown in size and is mature – it is called a Graafian follicle. An LH surge occurs and increases collagenase activity. This is an enzyme that disrupts collagen. Therefore, there is a weakening of the follicular wall. This, combined with muscular contractions of the ovarian wall, results in the ovum being released from the ovary. The ovum is then taken up into the fallopian tube via the fimbriae (finger-like projections of the fallopian tube). Fertilization – the Final Stage of Female Gametogenesis The secondary oocyte will only complete meiosis II following fertilization. Here, it gives off a third polar body. Following meiosis II, a fertilized egg results. If fertilization doesn’t occur, the oocyte degenerates 24 hours after ovulation, remaining arrested in meiosis II. If fertilization does occur, peristaltic movements of the fallopian tube move the egg to the uterus where it can implant into the posterior uterine wall. Prenatal Development The gestation period consists of three trimesters, each 3 months in duration: First trimester Cleavage Implantation Placentation Embryogenesis Second trimester Most organs finish development Third trimester Rapid growth Prenatal Development Cleavage and Blastocyst Formation Stages in the Implantation Process Stages in the Implantation Process Blastodisc Organization and Gastrulation The Fates of the Primary Germ Layers Ectoderm deratives ………………………….. Endoderm deratives ……………………………... Mesoderm deratives ……………………………… The Embryonic Membranes and Placenta Formation The Embryonic Membranes and Placenta Formation The Embryonic Membranes and Placenta Formation A Three-Dimensional View of Placental Structure: (a) Diagram of Placental Organization The First Trimester The First Trimester The First Trimester The First Trimester Extra Embryonic Mesoderm Extra embryonic mesoderm [EEM] splits to form extra embryonic coelomic cavity which is connected by connecting stalk[connecting stalk forms primary umbilical cord] Amnion It covers amniotic cavity and is Contributed by Amniogenic cells [from trophoblast] and Somatopleuric layer of EEM. Chorion It is contributed by cytotrophoblast that is Syncytiotrophoblast and somatopleuric layer of EEM Extra Embryonic Coelomic Cavity It divides the extra embryonic mesoderm into two parts. One towards the yolk sac is called as visceral or splanchnopleuric extra – embryonic mesoderm and beyond that lines the amniotic cavity and outside is called the parietal or somatopleuric extraembryonic mesoderm (towards body wall). Between somatopleuric extraembryonic and visceral/ splanchnic mesoderm is the extra embryonic coelomic cavity. Chorion formed by trophoblast with a somatopleuric layer of extraembryonic mesoderm. Amnion formed by amniogenic cells lining the amniotic cavity and somatopleuric layer of extraembryonic mesoderm. Connecting stalks later become umbilical cord components. Yolk Sac A membranous sac linked to an embryo called the yolk sac is made up of cells from the hypoblast layer of the bilaminar embryonic disc. The yolk sac is a far more common name for this, however the Terminologia Embryologica (TE) also calls it the umbilical vesicle. The yolk sac plays a crucial role in the early blood supply of human embryos, and during the fourth week of embryonic development, a large portion of it is integrated into the primordial gut. 1° YOLK SAC → lined by flattened Endodermal cells 2° YOLK SAC → lined by cuboidal Endodermal cells 3°YOLK SAC → formed during cephalo caudal development of Embryo →forms gut tube → Part of the yolk sac remaining outside the Embryo - 3 YOLK SAC In the beginning, we had a primary yolk sac. Later it will become secondary yolk sac and then tertiary yolk sac. Tertiary yolk sac communicates with midgut as vitello – intestinal duct. Extra – Embryonic coelomic cavity forming around the baby, there is cephalo- caudal folding of baby. Head comes towards the tail and in this process an amniotic cavity surrounds the body of the baby all around. Allantois [Hindgut diverticulum] and the Vitello intestinal duct [midgut diverticulum] will enter the umbilical cord to become its contents PLACENTAL COMPONENTS Placenta formed by fetal & maternal contributions Placenta formation: Decidua basalis from the maternal side and from the fetal side is chorion frondosum. Chorion layer develops some villi called chorionic villi and they will penetrate into decidua basalis of the maternal side. Maternal placenta is decidua basalis [endometrium of uterus] and chorionic villi from fetal placenta component UTERUS MATERNAL PLACENTA DECIDUA BASALIS [DB] → The endometrium where the embryo implants→forms the maternal /uterine placenta DECIDUA CAPSULARIS → Surrounds the embryo on luminal side DONOT FORM PLACENTA DECIDUA PARIETALIS →The rest of the gravid endometrium FETAL PLACENTA Derived from chorion CHORION FRONDOSUM - chorion towards DB forms layer like projections into it CHORION LAEVE - chorion on the side of D. capsularis, DO NOT FORM PLACENTA Uterine cavity = Space b/w decidua basalis & Parietalis Chorionic cavity = Chorion & Amnio Chorionic Villi The chorionic villi are small finger-like extensions of placental tissue that have the same genetic makeup as the developing foetus. Depending on the family history and the availability of lab testing at the time of the surgery, testing may be available for further genetic abnormalities and illnesse PRIMARY VILLUS [Day 12] Core of cytotrophoblast cells, covered by syncytio TB SECONDARY VILLUS [Day 13-15] Cytotrophoblast layer invaded by extra Embryonic mesoderm TERTIARY VILLUS [Day 17-21] Fetal blood vessels invades the mesoderm