Gametogenesis, Fertilization, & First Three Weeks of Human Development PDF

INTRODUCTION TO GAMETOGENESIS, FERTILIZATION, 1ST 3 WEEKS 2024 MSPC 112 – L2 BY FREDERICK K. ADDAI, PH.D., FWACM DEPT. OF ANATOMY, UGMS POSITIVE THOUGHT fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 2  STUDENT’S LEARNING OUTCOMES YOU SHOULD B...

INTRODUCTION TO GAMETOGENESIS, FERTILIZATION, 1ST 3 WEEKS 2024 MSPC 112 – L2 BY FREDERICK K. ADDAI, PH.D., FWACM DEPT. OF ANATOMY, UGMS POSITIVE THOUGHT fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 2  STUDENT’S LEARNING OUTCOMES YOU SHOULD BE ABLE TO UNDERSTAND THE FOLLOWING AT THE END OF THIS LECTURE. 1. HOW SPERMATOZOA ARE PRODUCED (GAMETOGENESIS) 2. HOW OOCYTES ARE PRODUCED (OOGENESIS) 3. WHAT FERTILIZATION IS, ITS IMMEDIATE CONSEQUENCES 4. THE PROCESSES IN THE FIRST, SECOND, AND THIRD WEEKS OF HUMAN DEVELOPMENT fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 3  GAMETOGENESIS IN THE MALE (SPERMATOGENESIS) GAMETES OR GERM CELLS IN MALES ARE SPERMATOZOA (SING. SPERMATOZOON) HENCE GAMETOGENESIS IN THE MALE IS KNOWN AS SPERMATOGENESIS SPERMATOGENESIS OCCURS IN THE TESTES AFTER PUBERTY (12-14 YEARS) UNDER INFLUENCE OF FOLLICLE-STIMULATING HORMONE (FSH) AND TESTOSTERONE A cartoon showing the hormonal fkwakuaddai/MSPC 112/2024 -L2 influence on spermatogenesis 04/06/2024 4  SPERMATOGENESIS OPTIMUM AT 2-3OC BELOW BODY TEMPERATURE (37OC) REASON THE TESTES ARE IN SCROTUM BELOW THE PELVIS (OUTSIDE ABDOMEN, UNLIKE OVARIES) CONTINUOUS THROUGHOUT POST-PUBERTAL LIFE PEAKS AT ABOUT AGE 25 YEARS ABOUT 150,000 SPERMATOZOA PER TESTIS PER DAY AT PEAK fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 5 fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 6  SPERMATOGENESIS - SUMMARY FROM SPERMATOGONIUM TO PRIMARY SPERMATOCYTES TAKES 16 DAYS FROM PRIMARY SPERMATOCYTES TO SPERMATIDS TAKES 24 DAYS FROM SPERMATIDS TO SPERMATOZOA TAKES 24 DAYS THE WHOLE PROCESS TAKES 64 DAYS fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 7  GAMETOGENESIS IN THE FEMALE (OOGENESIS) GAMETES IN FEMALES ARE SECONDARY OOCYTES (COMMONLY CALLED OVA; SING. OVUM) GAMETOGENESIS IN FEMALES IS KNOWN AS OOGENESIS OCCURS IN OVARIES AND COMMENCE BEFORE BIRTH (5 MONTHS IN UTERO) UNDER THE INFLUENCE OF FSH AND LUTEINIZING HORMONE (LH) fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 8  OOGENESIS –COMMENCES BEFORE BIRTH GAMETOGENESIS IN FEMALE IS PECULIARITIES OF OOGENESIS: OOGENESIS BECAUSE IT COMMENCES –– STARTS BEFORE BIRTH (5TH MONTH IN UTERO) WITH OOGONIA (SING. OOGONIUM) –– SUSPENDED IN THE MIDDLE (BIRTH TO PRIMORDIAL GERM CELLS (PGCS) PUBERTY) CHANGE TO OOGONIA, PRIMARY –– RESTARTS AT PUBERTY (9–13 YEARS) OOCYTES, AND SECONDARY OOCYTES (OR POST-PUBERTAL OOGENESIS IS TIED WITH THE OVA) MENSTRUAL CYCLE OCCURS IN OVARIAN CORTEX –– CONTINUES UP TO MENOPAUSE (45–55 YEARS) fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 9  OOGENESIS –THE EVENTS AFTER PUBERTY FOR 14 DAYS AFTER EVERY MENSTRUAL CYCLE SOME PRIMARY OOCYTES RESUME FIRST MEIOSIS TYPICALLY, ONE PRIMARY OOCYTE COMPLETES THE FIRST MEIOTIC DIVISION TO BECOME A SECONDARY OOCYTE (OVUM) UNDER THE INFLUENCE OF LH SURGE A MATURE OVARIAN FOLLICLE RUPTURES TO DISCHARGE THE SECONDARY OOCYTE (OVUM) FROM OVARY – PROCESS IS CALLED OVULATION THE SECONDARY OOCYTE IS SUCKED INTO FALLOPIAN TUBE SECOND MEIOTIC DIVISION IS COMPLETED ONLY AFTER IT HAS BEEN FERTILIZED BY A SPERMATOZOON fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 10  FERTILIZATION – FUSION OF ONE MALE GAMETE WITH ONE FEMALE GAMETE REQUIREMENTS FOR IN VIVO FEMALE OVULATION – SECONDARY FERTILIZATION OOCYTE (OVUM) MUST ENCOUNTER SUCCESSFUL COITUSINCLUDING RELEASE COMPETENT SPERMATOZOON WITHIN 24 OF SEMEN INTO VAGINA HOURS OF RELEASE ADEQUATE NUMBERS OF COMPETENT SECONDARY OOCYTE FREE FROM STRUCTURAL AND CHROMOSOMAL SPERMATOZOA ABNORMALITIES ABILITY OF SPERMATOZOA TO FEMALE GENITAL TRACT FREE FROM OVERCOME OBSTACLES IN THE FEMALE INFECTIONS AND APPROPRIATELY PRIMED BY GENITAL TRACT SEX HORMONES fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 11  SPERMATOZOON VS OVUM -SIZE SPERMATOZOON IS VERY SMALL AND CONTRIBUTES NO MORE THAN THE PATERNAL CHROMOSOMES OVUM IS LARGE AND SURROUNDED BY TWO LAYERS THERE IS A GELATINOUS INTERNAL LAYER KNOWN AS ZONA PELLUCIDA SEPARATING THE PLASMA MEMBRANE FROM AN OUTER LAYER OF CORONA RADIATA CELLS fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 12  SUMMARY GAMETOGENESIS COMPRISES SPERMATOGENESIS & OOGENESIS SPERMATOGENESIS COMMENCES AT PUBERTY, OCCURS AT LOWER-THAN- NORMAL BODY TEMPERATURE, DOES NOT STOP AFTER IT STARTS, AND PRODUCES MILLIONS OF SPERMATOZOA OOGENESIS IS SUSPENDED BETWEEN BIRTH AND PUBERTY, IT IS CYCLICAL, AND PRODUCES A SINGLE OVUM PER CYCLE THAT SUPPLIES CYTOPLASM FOR THE ZYGOTE fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 13  FERTILIZATION -OVERVIEW (1) Interaction between ovum and spermatozoa must result in one spermatozoon penetrating the oocyte resulting in fertilization Ovulation must release a secondary oocyte (ovum) that has completed the first meiotic division A cut-out diagram depicting pe ovulation, necessary for human fertilization 04/06/2024 Fkwakuaddai/MSPC 112/2024 -L2 14  FERTILIZATION -OVERVIEW (2) Early human development commences with the formation of a zygote –the end-product of fertilization The zygote undergoes a series of unique mitotic divisions called CLEAVAGE 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 15  FERTILIZATION -OVERVIEW (3) CLEAVAGE characterizes the first week of human development CLEAVAGE produces daughter cells called blastomeres Blastomeres for a ball of cells that is transported from the oviduct into the A cartoon depicting fertilization, and cleavage and transport from cavity of the uterus the oviduct into the uterine cavity 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 16  FERTILIZATION-OVERVIEW (4) The first 3-4 divisions do not involve the production of new protoplasm Cleavage starts with the 2-cell stage, then the 4-cell stage, 8-cell stage, 16-cell stage, etc. The ball of blastomeres at the 16-cell is called the MORULA 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 17  OBSTACLES THAT SPERMS ENCOUNTER IN THE FEMALE GENITAL TRACT 1. Acidity of vaginal fluid (many spermatozoa die) 2. Viscosity and acidity of cervical mucus (weaken and trap many spermatozoa, notably abnormal ones) 3. Distance from the vagina through uterine cervix and cavity, to the ampulla of the oviduct 4. Ref. https://doi.org/10.5935/MedicalExpress.2 015.02.07 5. Must survive 30 min to 12 hr swim and up to 72 hr wait for ovum 04/06/2024 18  FERTILIZATION REQUIRES A HEALTHY SECONDARY OOCYTE (OVUM) Devoid of structural abnormalities (which are very rare) Must have right number of chromosomes (22 autosomes + X) Must have completed 1st meiotic division prior to ovulation Must encounter competent sperm within 24 hours of ovulation Primary oocyte surrounded by corona radiata cells inside a mature follicle before ovulation 04/06/2024 19  FERTILIZATION – MULTISTEP PROCESS Occurs in three sequential events 1. Penetration of corona radiata 2. Penetration of the zona pellucida 3. Binding and Fusion of the oocyte and sperm cell membranes *Slow block to polyspermy or zonal reaction triggered by release of cortical granules from oocyte 04/06/2024 20  FERTILIZATION - MALE & FEMALE PRONUCLEI The sperm head releases paternal chromosomes into the oocyte to form the male pronucleus The secondary oocyte (ovum) completes the PB second meiotic division and ejects the PN PN second polar body (containing 23 chromosomes but with very little cytoplasm) The fertilized oocyte retains 23 maternal chromosomes that form the female pronucleus Fertilized oocyte showing 2 pronuclei (PN) and the second polar body (PB) 04/06/2024 21  IMMEDIATE EVENTS OF FERTILIZATION 1. Completion of second meiotic division of oocyte; formation of female pronucleus & expulsion of secondary polar body 2. Formation of the zygote by fusion of the male and female pronuclei 04/06/2024 22  IMMEDIATE EVENTS OF FERTILIZATION 3. Determination of genetic sex of the zygote (XX = female; XY = male) 5. Introduction of genetic 4. Restoration of the diploid variation (via cross-over & independent assortment of chromosomes) number of chromosomes (23 + 23 = 46) 6. Initiation of cleavage of zygote 04/06/2024 23  CLEAVAGE Occurs within oviducts No growth between divisions and the nuclear to cytoplasmic ratio decreases about 1000-fold Unusual cell cycles lacking or having very short, G1 and G2 phases A photomicrograph of two-cell stage blastomeres 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 24  WEEK ONE POST-FERTILIZATION Cleavage to implantation Cleavage - Succession of rapid divisions resulting in large number of smaller cells (blastomeres) No synthesis of new cytoplasm; thus, fractionation of oversize single cell (zygote) Blastomeres are enclosed within the zona pellucida – otherwise monozygotic twining FKWAKUADDAI/MSPC 112/2024 -L2 25 04/06/2024  THE HUMAN BLASTOCYST (DAYS 5-9 PF) Formation begins about 5 days after fertilization A fluid-filled cavity appears in the morula, the ball of 16 blastomeres Transported via “ciliary escalator” in oviduct to uterine cavity Arrives in uterine cavity day 7-9 pf 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 26  IMPLANTATION Process of attachment and invasion of the endometrium of uterus by the blastocyst (conceptus) Requires blastocyst to hatch out of the zona pellucida Trophoblast (outer cell mass) adheres to receptive endometrium (in secretory phase) A photomicrograph showing a hatching blastocyst 04/06/2024 27 FKWAKUADDAI/MSPC 112/2024 -L2  THE RECEPTIVE ENDOMETRIUM Corpus luteum (CL) formed from the ovarian follicle that discharged the secondary oocyte at ovulation CL under stimulation of LH produces progesterone Progesterone stimulates the endometrium to change from proliferating to secretory phase Blastocyst hatching must coincide with mid-secretory endometrium 04/06/2024 FKWAKUADDAI/MSPC 112/2024 -L2 28  SECOND WEEK OF DEVELOPMENT 1. Differentiation of cells during the formation of the blastocyst: Thus, as per the image shown; a. Embryoblast (inner cell mass (1) b. Fluid-filled (blastocyst) cavity called blastocoel (2) c. A single layer of outer cells that surround the blastocoel called trophoblast (3) Photomicrograph of a human blastocyst in a petri dish taken at a Fertility Laboratory 04/06/2024 29 fkwakuaddai/MSPC 112/2024 -L2  REVIEW -IMPLANTATION The process by which the blastocyst hatches out of the zona pellucida; its trophoblast adheres, attaches, and burrows into the endometrium where further development continues fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 30  THE SECOND WEEK OF HUMAN DEV’T Called the week of twos also because two cavities develop in relation to the 1 two layers of the embryoblast (The 2 bilaminar embryonic disc) fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 31 Week 3 of human development Characterized by replacement of bilaminar embryonic disc (epiblast and hypoblast) with a trilaminar disc Process of formation of trilaminar embryonic disc is called gastrulation All cell layers of trilaminar disc originate from epiblast via a primitive streak 04/06/2024 fkwakuaddai/MSPC 112/2024 -L2 32  GASTRULATION - Summary The Primitive Streak forms A Primitive node forms at cephalic from Epiblast end of primitive streak. One wave of Cells migrate to Cells of primitive node migrate below surface to form Notochord replace the hypoblast to form Endoderm Epiblast cells that did not form part of primitive streak or node, remain A second wave of cells as surface ectoderm. migrate to form a layer Thus, trilaminar embryonic disc results beneath the epiblast to form from gastrulation, comprising: Mesoderm Ectoderm Mesoderm Endoderm 04/06/2024 fkwakuaddai/MSPC 112/2024 -L2 33 EMBRYONIC ORIGIN OF BASIC TISSUES 04/06/2024 fkwakuaddai/MSPC 112/2024 -L2 34 GASTRULATION & THE FOUR BASIC TISSUES 1. Epithelia – arise from all the primary germ layers; ectoderm, mesoderm, and endoderm 2. Connective Tissues – arise exclusively from mesoderm 3. Nerve Tissue – Arise exclusively from ectoderm 4. Muscle Tissue – arise exclusively from mesoderm 04/06/2024 fkwakuaddai/MSPC 112/2024 -L2 35 THE END HAVE A GREAT EVENING! fkwakuaddai/MSPC 112/2024 -L2 04/06/2024 36