DEVELOPMENTAL BIOLOGY AND HUMAN WELFARE M.Sc Zoology II YEAR BHARATHIAR UNIVERSITY COIMBATORE – 641046 SCHOOL OF DISTANCE EDUCATION 1 2 DEVELOPMENTAL BIOLOGY AND HUMAN WELFARE TABLE OF CONTENTS UNIT TITLE PAGES I GAMETOGENESIS 4 II FERTILIZATION PROCESS 30 III EMBRYOGENESIS 43 IV EMBRYONIC INDUCTION 73 V HUMAN WELFARE 105 3 UNIT I GAMETOGENESIS Sexual reproduction involves the formation of male and female gametes and the mechanisms necessary for the gametes to come together and fuse to form one cell that represents the beginning of a new individual with a distinct genetic identity. Preparation for pregnancy involves two main programs of events: The process of formation of the male and female gametes occurs in the gonads (ovary or testis). SPERMATOGENESIS 1.0 INTRODUCTION 1.1 STRUCTURE OF THE GERMINAL EPITHELIUM 1.2 DEVELOPMENTAL STAGES OF SPERMATOGENESIS 1.3 THE TEMPORAL COURSE OF SPERMATOGENESIS 1.4 SPERMATOCYTOGENESIS 1.5 LOCAL COURSE OF SPERMATOGENESIS - THE SPERMATOGENESIS WAVE 1.6 SPERMIOGENESIS (SPERMATOHISTOGENESIS) AND STRUCTURE OF THE SPERM CELL 1.7 LEYDIG'S INTERSTITIAL CELLS AND HORMONAL REGULATION 1.0 INTRODUCTION Spermatogenesis is initiated in the male testis with the beginning of puberty. This comprises the entire development of the spermatogonia (former primordial germ cells) up to sperm cells. The gonadal cords that are solid up till then in the juvenile testis develop a lumen with the start of puberty. They then gradually transform themselves into spermatic canals. They are termed convoluted seminiferous tubules (Tubuli seminiferi contorti) and are so numerous and thin that in an adult male testicle. They are coated by a germinal epithelium that exhibits two differing cell populations: some are sustentacular cells (= Sertoli's cells) and the great majority of the germ cells in various division and differentiation. 4 Fig. Convoluted seminiferous tubules. For an optimal sperm cell production a certain milieu is needed. By transferring the testicles into the scrotum a testicular temperature 2-3 ºC lower than body temperature is attained. In addition, a slightly elevated pressure from the surroundings is necessary. This is why when the taut tunica albuginea is slit open, the testicular parenchyma bulges out by itself. Evidently, both elevated pressure and lowered temperature are necessary for producing sperm cells. 1. Basal lamina (membrane) (not recognizable) 2. Myofibroblast 3. Fibrocyte 4. Sertoli's cell 5. Spermatogonia 6. Various stages of the germ cells during spermatogenesis 7. Spermatozoon 8. Lumen Fig. Histological transverse section of a portion of convoluted seminiferous tubules in an adult. Outside its basal lamina a layer of myofibroblasts and fibrocytes surround the tubule. The germinal epithelium lies on the tubule wall. One can recognize the spermatogonia sitting on the basal lamina. The nuclei of the Sertoli's sustentacular cells 5 have a rarified chromatin and the nuclei with clear nucleolus that are often oriented perpendicular to the basal lamina. The overall picture, though, is dominated by the cells occupied with spermatogenesis. The development of the germ cells begins with the spermatogonia at the periphery of the seminal canal and advances towards the lumen over spermatocytes I (primary spermatocytes), spermatocytes II (secondary spermatocytes), spermatids and finally to mature sperm cells. 1.1 STRUCTURE OF GERMINAL EPITHELIUM The epithelium consists of Sertoli's sustentacular cells and the spermatogenic cells. The Sertoli's cells form a single-layered lamina and extend from the basal lamina to the tubule lumen. With their labyrinthine cellular processes they surround the individual types of germ cells more or less completely. Spermatogenesis is thus accomplished in close contact with the Sertoli's cells, which not only have supportive and nourishing functions, but also secrete hormones and phagocytize cell fragments. Somewhat above the basal lamina they are bound to each other through complicated occluding junctional complexes (tight junctions), so that 2 separated compartments are present in the epithelium: a basal one, in which the spermatogonia are lined up, and a luminal one, in which all the other stages of spermatogenesis are found. 1. Peritubular cells, 2. Basal membrane, 3. Spermatogonia, 4. Tight junction, 5. Spermatocyte I, 6. Spermatocyte II, 7a. Spermatids, 7b. Spermatids, 8.Acrosome, 9. Residual bodies, 10. Spermatozoas, 11. Cell nucleus of sustentacular cells (Sertoli), A. Basal zone, B. Adluminal zone 6 Fig. Germinal epithelium. Schema of the germinal epithelium: The supportive (Sertoli) cells sit on the basal membrane. Towards the lumen of the spermatogonia (lowest row of cells) the Sertoli cells are connected with each other by the occluding junctional complexes (tight junctions). This seal gives rise to the blood-testicle barrier. The cytoplasm of these supportive cells gets formed into complicated processes because they surround all of the cells involved with spermatogenesi. Through the occluding junctional complexes of the Sertoli's cells a "blood/testicle" barrier is created in the tubule. This means that outside this barrier, in the tubular periphery, cells, substances and hormones from the blood have unhindered access. On the other hand, the inner compartment of the tubule is protected by the barrier, which is selectively permeable and serves as an entry check. This is of practical importance because haploid cells in the inner part of the tubule exhibit surface antigenic properties, different from all other body cells. They must thus be kept secluded from the immune system of the organism by the "blood/testicle" barrier. 1.2 DEVELOPMENTAL STAGES OF SPERMATOGENESIS In the course of spermatogenesis the germ cells move towards the lumen as they mature. The following developmental stages are thereby passed through: • A-spermatogonium • B-spermatogonium • Primary spermatocyte (= spermatocyte order I) • Secondary spermatocyte (= spermatocyte order II) • Spermatid • Sperm cell (= spermatozoon) The spermatogenesis can be subdivided into two successive sections: The first comprises the cells from the spermatogonium up to and including the secondary spermatocyte and is termed spermatocytogenesis. 7 The second one comprises the differentiation/maturation of the sperm cell, starting with the spermatid phase and is termed spermiogenesis (or spermiohistogenesis). Fig. The spermatogenesis generations. The stem cell population of the germinal cells lies on the basal lamina of the convoluted seminiferous tubules. These are Type A spermatogonia. These cells undergo mitosis: one of the daughter cells renew the stock of type A spermatogonia, the other becomes a type B spermatogonia. These divide and their daughter cells migrate towards the lumen. In roughly 64 days they differentiate themselves thereby into sperm cells up to the outer surface of the epithelium (one should note that in these cellular divisions, the separation of the cytoplasm is not complete. Whole networks of connected cells arise. So, for example in the last generation, the spermatids, far more cells are bound to each other than as shown here). 1.3 THE TEMPORAL COURSE OF SPERMATOGENESIS The approximate 64 day cycle of the spermatogenesis can be subdivided into four phases that last differing lengths of time: 8 Mitosis of the 16 days Up to the primary spermatocytes spermatogonia First meiosis 24 days For the division of the primary spermatocytes to form secondary spermatocytes Second meiosis A few hours For engendering the spermatids Spermiogenesis 24 days Up to the completed sperm cells 1.4 SPERMATOCYTOGENESIS Among the spermatogonia (all in all, over 1 billion in both testicles) that form the basal layer of the germinal epithelium, several types can be distinguished: certain type A cells are seen as spermatogonia that divide mitotically and reproduce themselves (homonymous division), whereby the spermatogonia population is maintained. The beginning of spermatogenesis is introduced through the so-called heteronymous division, in which the daughter cells (second group of type A cells) remain bound together by thin bridges of cytoplasm. Through the preservation of these cytoplasmic connections, spermatogonia are inducted into the spermatogenesis process. After a further mitotic division type B spermatogonia are engendered that also divide themselves mitotically into primary spermatocytes (I). The freshly created primary spermatocytes (I) now enter into the first meiosis. They then go immediately into the S phase (that is, into the preleptotene meiosis), double their internal DNA, leave the basal compartment and reach the special milieu of the luminal compartment. Following the S phase, these cells attain the complex stage of the prophase of the meiosis and become thereby noticeably visible with a light microscope. This prophase, which lasts 24 days, can be divided into five sections: Leptotene , Zygotene, Pachytene, Diplotene and Diakinesis. In the prophase in every germ cell a new combination of maternal and paternal genetic material occurs. After the long prophase follow the metaphase, anaphase and 9 telophase that take much less time. One primary spermatocyte yields two secondary spermatocytes. In the heteronymous division the cytoplasmic division is not completed; the daughter cells stay bound together through thin cytoplasmic bridges. Also in the subsequent meiosis the cytoplasmic division is incomplete, so that from one spermatogonium a network of daughter cells arises that doubles in size in each generation.