Early Embryonic Development PDF

Early embryonic development (part 1) Spermatogenesis: - Produces spermatozoa in the seminiferous tubules of the testes Inside the lobes of the testis, there are seminiferous tubules (ST). Each ST contains Sertoli cells & primordial germ cell (which gives rise to sp...

Early embryonic development (part 1) Spermatogenesis: - Produces spermatozoa in the seminiferous tubules of the testes Inside the lobes of the testis, there are seminiferous tubules (ST). Each ST contains Sertoli cells & primordial germ cell (which gives rise to spermatogonia) Leydig cells are located in the interstitial space between ST - Spermiogenesis (final stage of spermatogenesis): Transformation of immotile spermatids into motile spermatozoa (sperm cells) Structural changes that occur in spermiogenesis: 1. Formation of acrosomal cap 2. Condensation of nucleus 3. Formation of the neck, middle piece & tail 4. Shredding of most cytoplasm Structure of spermatozoa: ◼ Acrosome contains acrosomal enzymes ◼ Mitochondria provides energy for movement of the tail Phases: Primordial germ cell→spermatogonium→primary spermatocyte→ secondary spermatocyte→ spermatid→ spermatozoa Gender differences: this process is continuous throughout a man reproductive life Hormones involved in spermatogenesis: Paste pg10 1. Hypothalamus releases GnRH 2. GnRH binds to receptors on gonadotropes (in the anterior pituitary gland) & stimulate the release of LH & FSH 3. LH acts on leydig cells to secrete testosterone 4. FSH binds to receptors on Sertoli cells & stimulates production of androgen-binding protein & inhibin proteins Sertoli cells: nourish & support the maturation of spermatozoa ABP: testosterone enters seminiferous tubules & binds to ABP, which makes testosterone less lipophilic & more soluble Inhibin: acts on the hypothalamus, provide negative feedback to hypothalamus to regulate the production of GnRH  Oogenesis: - Occurs in follicles in the cortex of the ovary - Starts b4 birth at 4 weeks of embryo, then arrested in diplotene stage of prophase 1 meiosis in the primordial follicle. At puberty, oogenesis resumes & ends at fertilization Chromosomal anomaly during meiosis 1 & 2: Nondisjunction Anaphase lag - Splitting of chromosome at the centromere during anaphase of mitosis & meiosis called disjunction - One part of the chromosome moves to the pole but another part can’t - In non-disjunction, a complete chromosome moves to one pole & the other pole gets no chromosome move, it’s lost from cell due to failure of action of microtubule - Results: one cell trisomy & other cell monosomy - Results: one cell normal & other cell monosomy - Down syndrome (trisomy 21), extra copy of chromosome 21 (making 3 copies instead of normal 2), occurs in meiosis-2 - Turner syndrome, instead of the normal two sex chromosomes (XX), individuals with Turner syndrome have only one X chromosome, resulting in monosomy X - Primordial germ cell (from wall of yolk sac) differentiates into oogonia - Oogonia undergo meiosis forming primary oocyte - End of 3rd month, oogonia are arranged in clusters surrounded by a layer of epithelial cells- primordial follicle Stages of development of primordial follicle: 1. Primary or preantral follicle 2. Secondary or antral follicle 3. Pre-ovulatory graafian follicle Primary or preantral follicle Paste pic from goodnotes pg20 - Primary oocyte: mitosis 1. Oogonia undergo meiosis to form primary oocytes 2. Meiotic division arrests at diplotene stage of prophase 1 until the fetus is born & grows to puberty 3. In each menstrual cycle, a few oocyte follicles will develop- one will develop into graafian follicle to release ovum during ovulation - (Primordial follicles are present at birth and remain in a dormant state in the ovaries throughout childhood, persisting until puberty ) inactive) temporary Secondary or antral follicle Paste pic from goodnotes pg20 - LH acts on theca cells, stimulates theca cells to take in cholesterol & convert it into androgens. Androgens move into granulosa cells. FSH stimulates granulosa cells to convert androgen into estrogen. - Primary oocyte completes meiosis 1→secondary oocyte + polar body Pre-ovulatory Graafian follicle Paste pic pg19 - Secretes estrogen, works on the endometrium of the uterus (for implantation) - Secondary oocyte starts meiosis 2→arrests in metaphase 2 - The secondary oocyte remains in this phase until fertilization occurs. If fertilization occurs, the secondary oocyte will complete meiosis 2, producing mature ovum  Hormones Paste pg23 1. Oestrogen works on the endometrium of the uterus & maintains the proliferative phase of the endometrium 2. Progesterone: maintains the implantation of the endometrium lining to prevent shredding of the endometrium lining After the graafian follicle develops: 1. Ovulation: - During graafian follicle: Estrogen lvl rises due to maturation of the dominant follicle (refers to the development of the graafian follicle & secondary oocyte within it) - Positive feedback: increasing estrogen lvl enhance the release of LH, leading to LH surge. LH surge triggers ovulation, causes the rupture of the follicle & the release of secondary oocyte into the fallopian tube (uterine tube) - Corona radiata: granulosa cells that’re attached to zona pellucida 2. Corpus luteum: - After ovulation, the remnants of graafian follicle forms the corpus luteum - Func of corpus luteum: produce progesterone & estrogen - 2 types: pregnancy, menstruation I. Corpus luteum of pregnancy: - If fertilisation & implantation occur, CL wld be persistent for 3 months, continues to func. & secrete hormones until the placenta is fully developed II. Corpus luteum of menstruation: if implantation doesn’t occur, CL persists for 9 days then breakdown, leading to decrease in progesterone which triggers menstruation Pre-requisition of fertilization: Phase 1: penetration of the corona radiata 1. Capacitation - Site: occurs in the uterine tube - Glycoprotein coat & seminal plasma proteins are removed from the plasma membrane that overlies with the acrosomal region of spermatozoa - This process leaves a specific modified glycoprotein, becomes hypermotile. Capacitated sperm can move quickly thru the hyaluronic acid (follicular fluid inside the antrum) into the corona radiata→the capacitated sperm move towards ZP3 receptor molecules n zona pellucida - Only capacitated sperm can pass freely thru corona cells & undergo acrosomal reaction 2. Fast block to polyspermy - A rapid mechanism that prevents multiple sperm from fertilizing the same egg - 1st binding: sperm binds to zona pellucida type 3 receptors (ZP3), Ca2+ enters the sperm head - 2nd binding: the vesicle of the acrosome fuses with ZP3, enzymes (acrosin) are released from acrosomal cap & penetrate thru the zona pellucida 3. Acrosomal reaction - Once penetrated thru the zona pellucida, spermatozoa initiate its entry into the oocyte membrane - Sperm binds to beta subunit of the egg membrane (oocyte membrane) - Binding activates channels that allw Na+ to flow into the oocyte - The flow of Na+ creates a positive charge across the egg’s membrane, which prevents more sperms from binding to the beta subunit (fast block to polyspermy) - Cortical reaction: The sperm then binds to the alpha subunit, causing the sperm head to fuse with oocyte membrane. This binding triggers the cortical reaction, releasing cortical granules that contains  lysosomal enzymes modify the zona pellucida, resulting in hardening of this layer, prevents additional sperms form entering (slow block polyspermy) - At cortical reaction, secondary oocyte completes 2nd meiotic division & becomes an ovum 4. Slow block to polyspermy: - Once the sperm enters the oocyte, Ca2+ is released from the smooth ER of the oocyte - Lysosomal enzymes in oocyte binds to the oocyte’s cell membrane & release hydrolytic enzymes (breakdown of zona pellucida) - Zonal reaction: leads to hardening of the zona pellucida, no further sperm can gain access to the oocyte (still in metaphase 2, going to complete 2 nd meiotic division to form mature ovum) Summary: the 3 barriers spermatozoa needs to cross to fertilize an ovum: - Corona radiata - Zona pellucida - Oocyte membrane 5. Fertilisation: - Mature ovum & male gamete fuses to form zygote - Fertilisation 1. Penetration of corona radiata 2. Penetration of the zona pellucida 3. Fusion of the oocyte and sperm cell membranes - results of fertilization: - restoration of the diploid nmb of chromosome (half from father & half from mother) - determination of the sex of new indiv. → An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. -initiation of cleavage The early stages of embryonic development (cleavage & implantation) Paste pic pg20 from goodnotes Blastulation: - Morula transforms into blastocyst - Outer cell mass: trophoblast (forms placenta)→ cytotrophoblast & syncytiotrophoblast Cytotrophoblast: single nucleus & hv definitive (clear) cell boundary Syncytiotrophoblast: cell merge tgt to form a big mass of cells with many nuclei, no definitive cell boundary - Inner cell mass: embryoblast (forms embryo)→bilaminar disc→epiblast & hypoblast Implantation: - Embedding of the blastocyst in the functional layer of endometrium anterior or posterior wall of the uterus close to the fundus (upper part of the uterus) 2nd week of development: formation of the bilaminar germ disc & trophoblast (cyto & syncytiotrophoblast) Syncytiotrophoblast ◼ Func.: invades the endometrium to create lacunae (spaces that fill with maternal blood) 1. After implantation, syncytiotrophoblast begins to secrete beta-hCG that signals the corpus luteum to keep producing progesterone to prevent shredding of the endometrial lining  2. Syncytiotrophoblast releases hydrolytic enzymes (which incudes proteolytic enzymes) to make its way thru the uterine lining 3. This uterine lining (also referred as endometrium) has maternal blood vessels & as the syncytiotrophoblast move deeper into the endometrium, vacuoles are formed within its structure, these vacuoles fuses tgt to form lacunae. It fuses with the maternal blood. 4. The fusing of syncytiotrophoblast + maternal blood supply. Lacunae is filled with maternal blood, enabling the transfer of nutrients & oxygen from the mother to the developing fetus. (maternal circulation is connected with placental circulation) this develops the formation of placenta. ◼ Bilaminar germ disc: epiblast & hypoblast ➔ Epiblast: consists of columnar cells located adjacent to the amnioblast that forms the amniotic cavity. Epiblast gives rise to 3 germ layers: ectoderm, mesoderm, endoderm thru the process of gastrulation in the 3rd week of embryonic development. These germ layers form the entire embryo ➔ Hypoblast: forms yolk sac & lining of the chorionic cavity Abnormal sites of implantation: - Ectopic pregnancy: implantation outside normal position When blastocyst implants outside of the uterus Etc: tubal pregnancy ◼ Implantation of blastocyst in the fallopian tube. When rupture occurs, women experience normal menstruations in the early stages, therefore she won’t know if she’s pregnant - Hydatidiform mole (molar pregnancy) Trophoblast develops abnormally, no embryonic tissue  Primary or preantral follicle Syncyiotrophoblast secrete hcG- signalc(to produce progesterone to prevent shredding of the ~ endometrial lining invades endometrium by: releasing hydrolytic - granulosa cells enzymes ⑪ flat follicular cells ③ to invade uterine lining Esse primary oocyte Lendometrium) soy primary ofyear 8 Zona pellucida I glycoprotein layer forms vacuoles sur oundinganee fuses ↳ the microvilliof oocyte -vacuoles 191, projects into zona pellucida forms lacunae ssscuboidalfollicular e to facilitate the transfer ② of nutrients from the I lacunae filled with to the granulosa cells primary oocyte maternal blood Zona pellucida I placenta formed follicle primary Secondary or antral follicle Theca externa:outer fibrous layer vlaro in van erasezona glabrocides s.owner a jointgy, forming, fluid. sipedcarily penicset The early stages of embryonic development (cleavage & implantation) nreceptor - ZP3 ⑧ The sperm binding to 2P3- receptor causes Ovum the entry of nucleus Zygote metaphase ⑲ ⑧ anaphase >blastulation -Prophoblast contercell mass) ⑳800 -> ⑧8 - embryoblast linner cell mass) ⑭ cleavage:repeated mitotic division of the zygote - rapid increase in Blastocyst the umb of cells Process of fertilization: 3 phases: Phase 1:penetration of the corona radiata primordial follicle Phase 2: penetration of the zona pellucida Dogonia undergo meiosis to form primary oocyte - Phase 3: Fusion of the sperm cell with the oocyte membrane Meiosis is arrested atdiplotene stage of prophase I - before birth until ⑪ Capacitation puberty glycoprotein coat & seminal plasma proteins removed they - as overlaps with the acrosomal region of the spermatozoa Primary follicle I -capacitated sperms moves thru the quickly hyaluronic acid (follicular fluid inside the antrum Secondary follicle -LH acts on theca cells, stimulates theca cells to take in cholesterol, into the corona radiata converted to: Cholesterol Androgens- -Capacitated sperm move towards 2P3 receptor in zona pellucida move into granulosa cells - in the granulosa cells, FSH stimulates granulosa cells to convert androgenstestrogen only capacitated sperm pass thru corona radiated acrosomal reaction * can Meiosis I is completed:forms secondary oocyte+1 polar body - Pre-ovulatory grantian follicle ② fastblock to -> polyspermy -estrogen works on the endometrial lining (prepares implantation) -prevents multiple sperm from fertilizing the same egg -starts meiosis 2 - arrested at metaphase 2 - Meiosis 2 Is binding:sperm binds to 2P3, Ca enters sperm head only progress if fertilization occur, producing an ovum 2nd binding: vesicles of the acrosomal cap binds to 2P3, cause release of acrosin from acrosomal cap & penetrate thru zona pellucida ① Ovulation ③ Acrosomal reaction -During graatian follicle, estrogen (v) rises due to maturation of the dominant follicle Positive feedback: causes LH Sperm binds to B-subunit of increasing estrogenlul surge. - oocyte membrane -this LH surge triggers ovulation:breakdown of the follicle binding activates Natto flow into the orcyte the flow of Wat creates a &release of 2oocyte '+' charge across the oocyte sperm from binding - membrane, prevents more to the B-subunit into the fallopian tube corona radiata:granulosa cells surrounding the zona pellucida - Cortical reaction:the slow block to sperm then binds to the alpha subunit polyspermy: ② Corpus luteum sperm head fuse with the oocyte membrane, - cortical granules that ↳ Produces & release of (contains lysosomal enzymes), estrogen progesterone hardens the zonapellucida, prevents additional sperm from entering (slow block to polyspeny) (L if for 31 pregnant:persist - 2 oocyte completes meiosis 2, forms mature ovum -function until the placenta - once the spermenters the oocyte, Carti s released is fully developed lysosomal enzymes in oocyte binds to oocyte membrane - release hydrolytic enzymes C2 if menstruation; persist for 97 · - Zonal reaction:leads to of zonapellucida, no furthersperm can enter -breakdown hardening orcyte - -I in progesterone triggers menstruction Endometrial cycle Cluteal phasel iimportance: l - endometrium increases in height, -few ladies secretory phase tochous, artery more nutrients to didn't get gives endometrium the ↳ Check endometrium - notthisaction embryo Thickness Eilisation there--not there ↳ abolition