General Embryology Lecture Notes PDF

GENERAL EMBRYOLOGY by Dr.Haidy Refaat  INTRODUCTION TO EMBRYOLOGY DEFINITIONS Embryology is the study of embryos (G.embryon=is the early developing human). It refers to the prenatal development of embryos and fetuses. Developmental anatomy is concerned with the change...

GENERAL EMBRYOLOGY by Dr.Haidy Refaat  INTRODUCTION TO EMBRYOLOGY DEFINITIONS Embryology is the study of embryos (G.embryon=is the early developing human). It refers to the prenatal development of embryos and fetuses. Developmental anatomy is concerned with the changes that cells, tissues, organs and the body as a whole undergo from germ cells to the adult. Teratology (teratos=monester) is the division of embryology that deals with birth defects..  Developmental periods 1-Prenatal period: a)Embryonic period: from fertilization to the end of the 8th week. b)Fetal period: from the end of the 8th week to birth. 2-Postnatal period: Newborn or neonate: period of the first four weeks (first month) of postnatal life. Infancy: first year of postnatal development. The body as a whole grows rapidly during the first year; total length increases by one-half and weight is tripled. 3-Childhood: from end of firs year to puberty. SPERMATOGENESIS Definition: Spermatogenesis is the steps by which spermatogonia are transformed into spermatozoa. Site: Seminiferous tubules of testes. Duration: 2 month. Steps: A-Spermatocytogenesis B-Spermiogenesis Steps: A-Spermatocytogenesis - At puberty, the primordial germ cells, in seminiferous tubules of testes, start differentiation into spermatogonia. -Spermatogonia are the basal cells; they are of two types; A cell and B cell. A cells are stem cells that undergo mitosis to provide a continuous reserve of stem cells and a more differentiated B cells. B cells divide by mitosis into 2 primary spermatocytes. -Primary spermatocytes are large cells that migrate toward the middle zone of the seminiferous tubules and undergo a prolonged first meiotic division (meiosis I) to form two smaller secondary spermatocytes, with half number of chromosomes (23 X or Y).  Secondary spermatocytes divide by mitosis (meiosis II) to form two smaller cells, spermatids. -Spermatids become embedded in Sertoli cells which provide support, protection and nutrition for germ cells. B-Spermiogenesis: It is the series of changes resulting in transformation of spermatids into spermatozoa. This process includes: 1-The nucleus will form head of sperm. 2-Golgi apparatus form acrosomal cap over the surface of the head of sperm. Acrosome contains enzymes that assist penetration of the ovum. 3-The Centrioles move to the side of the nucleus opposite acrosomal cap &gives rise to an axial filament that grows out and penetrates the cell surface. 4-Mitochondria migrate toward the axial filament to form mitochondrial sheath that lies in the middle piece (body) of the spermatozoon. 5-The cytoplasm is cast off from the developing spermatozoon. Mature spermatozoon is formed of oval head, neck, middle piece and tail. Each spermatogonium gives rise to 8 spermatozoa (haploid number of chromosomes, 23 X or Y). Notes 20 days are required for spermatozoa to traverse the 6 meter coiled duct of epididymis. The seminal fluid (semen) is the spermatozoa and secretions of accessory sex glands. Spermatozoa account for only 10% of the seminal fluid. The remainder of the ejaculate consists of secretions of the seminal vesicles (60-80%), prostate (20%) and bulbourethral glands (10%). -Volume of ejaculate is 3-5 ml. -Immotile and abnormal forms of sperms usually are less than 25% of the total number. -For potential fertility at least 40% of sperms should be motile -FSH secreted by anterior pituitary activates spermatogenesis. Fertility ability of sperms is one-two days. Testis and drainage ducts Abnormal seminal fluid analysis may show: Azospermia: absence of spermatozoa in semen. Oligospermia: deficiency in sperm number (less than 20 million/ml). Necrospermia: sperms are dead or immotile. Athenospermia (Reduced motility) of sperms as in varicocele. Excessive number of abnormal forms: giant or dwarf sperms, bifid head or tail, tapering head, too small or too large head and joined sperms. Abnormal sperms move in circles or in irregular pattern. Definition: is sequence of events by which OOGENESIS oogonia are transformed into mature oocytes. Steps: 1-Prenatal Maturation -Primordial germ cells, arrived at the ovary from yolk sac, differentiate into oogonia. By the 3rd month of gestation, oogonia undergo mitosis to give nests of oogonia. Oogonia are surrounded by a layer of flat epithelial cells, known as follicular cells, originating from surface epithelium covering the ovary. A primary oocyte together with its surrounding flat epithelial cells is known as primordial follicle. - By 5th prenatal month, total number of germ cells in the ovary reaches its maximum, 7 million. --By the 7th month the majority of oogonia degenerate All surviving primary oocytes have entered the first meiotic division,. The total number of oocytes at birth is one million. 2-Postnatal Maturation -From birth to puberty 90% of primordial follicles degenerate. By time of puberty there are only about 40,000 primordial follicles in each ovary. Only about 400 primary oocytes become secondary oocytes that are expelled at ovulations. Ovulation is necessary for the first meiotic division to be completed. --During ovulation primary oocyte undergoes first meiosis to give rise to secondary oocyte and a small first polar body, each with haploid number of chromosomes. The first polar body degenerates. -Secondary oocyte enters the second meiotic division (meiosis II) without DNA replication. The second meiotic division is only completed if the secondary oocyte is fertilized. Secondary oocyte will give rise to mature ovum (ootid) and a second polar body, each with haploid number of chromosomes. Stages of oogenesis  Comparison between male and female gametes Male gamete Female gamete Gametogenesis Primary spermatocyte No primary oocytes form after continuously formed. birth. Size Sperm is smaller 60 m. Ovum is larger 140 micron. Motility Sperms are motile. Oocytes are immotile. Surroundings No. Zona pellucida and corona radiata Sex chromosomes Either X or Y. Only X. Cytoplasm Sparse. Abundant. Abnormal oocytes -Oocytes with 2 or 3 nuclei. -Some ovarian follicles contain 2 or more oocytes. -Non-disjunction; trisomy or monosomy. -Risk factors for gamete abnormalities for women increase with age (above 35 years), use of some drugs, and exposure to irradiation or viruses. THANKS