Lec40n - Lecture 40 Gametogenesis fertilization and early...

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Lecture 40 Gametogenesis, fertilization, and early development Campbell 6 th Ed. Chap. 46 (pg. 9284-986) & Chap. 47 (999-1006; 1010-1011) 7 th Ed. Chap. 46 (969-971 & 973-975) & Chap. 47 (988-996; 999-1000) 8 th Ed. scanned pages 974-975 (Figs. 46.11 + 46.12) & Ch. 47: 1021-1023; 1025-1030, 1033, 1034 In the last 3 lectures in the course, we’ll discuss Development; the process of turning a single- celled zygote into a multicellular organism. This is one of the most actively-studied areas at the forefront of biology today. Today we’ll talk about formation of gametes and fertilization, and outline the steps in early development of animals. Gametogenesis in humans: Gametogenesis is formation of gametes. Spermatogenesis is formation of sperm, and oogenesis is formation of eggs. Spermatogenesis - Sperm develop in seminiferous tubules, tightly packed tubules that make up most of the testes (Fig. 46.11/46.12; scanned on Blackboard). Each tubule has a thick wall and a lumen or hollow center. The walls are made up of developing sperm. The cells at the outer part of the wall are the most immature. As the cells mature, they move inwards and are eventually released into the lumen. Only mature sperm are released; the other cells are all present in the tubule walls. Meiosis is coupled to gametogenesis - Remember that in mammals, meiosis only occurs in developing gametes. As developing sperm move inwards in the wall of the seminiferous tubule, they go through meiosis at the same time that they differentiate from round precursor cells to mature cells that have the characteristic shape of sperm. The diploid precursor cells at the outer edge of the tubule wall are called spermatogonia (singular; spermatogonium). They are the only cells in the male that are capable of dividing both by mitosis (to make more of themselves) and by meiosis, as they start differentiating to sperm. Because spermatogonia keep dividing by mitosis, males continue to make sperm throughout life. Cells that enter meiosis spend a relatively long time in Prophase I. These cells (in Prophase I) are called primary spermatocytes . After they finish Meiosis I and divide, they are called secondary spermatocytes . These enter Meiosis II and divide to form spermatids . These are haploid cells, although they still have a round shape. They differentiate further (with no changes in their chromosomes) to give mature sperm. Each spermatogonium produces 4 sperm as it goes through meiosis. Mature sperm are released into the lumen at the center of the tubule, and pass out of the testes, up into the abdominal cavity through tubes called the vas deferens, and are then released through the penis during ejaculation. Oogenesis
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Lec40n - Lecture 40 Gametogenesis fertilization and early...

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