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EARLY MAMMALIAN DEVELOPMENT
Mammalian development begins with fertilization inside the female's body. The zygote then undergoes cleavage divisions that are slower than in other animals. Around the 8-cell stage, the cells compact together and later form two distinct cell types - the inner cell mass and trophoblast cells. The trophoblast cells go on to form the blastocyst, with an inner cell mass surrounded by trophoblast cells. The blastocyst hatches from the zona pellucida and implants in the uterus, where the trophoblast cells invade the uterine tissue and the inner cell mass forms the embryo.
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EARLY MAMMALIAN DEVELOPMENT
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- 2. Name Arsalan aslam4602 Sahar ali 4606 S.Ashal Fatima 4623 Topic of presentation Early mammalian development Presented to Ahmad Raza Azam
- 3. • Contents • Earlymammalian development • Unique nature of mammalian cleavage • Fertilization • Type of cleavage • Early genome activation • Stages of development • Compaction • Morula stage • Cavitation and blastocyst formation • Escape from zonapellucida • Implantation
- 4. Early mammalian development 1.Smallin size • Mammalian eggs are among the smallest in the animal kingdom. • The human zygote is only 100µm in diameter and hardly visible to the eye. 2.Less in number • Mammalian zygotes are produced less in number. • Fewer than ten eggs are ovulated by a female at a given time. 3.Internal fertilization • The development of mammalian embryos is accomplished internally rather than in the external environment.
- 5. The unique natureof mammalian cleavage • Mammalian cleavage is different from most other patterns of embryonic cell division. • Before fertilization the mammalian oocyte is released from the ovary and swept by the fimbriae into the oviduct.
- 6. a) Fertilization • Fertilizationoccurs in the ampulla of the oviduct, a region close to the ovary. • Meiosis is completed after sperm entry • First cleavage begins about a day later.
- 8. b) Slowest cleavages •Cleavage in mammalian eggs are among the slowest in the animal kingdom (12-24 hours). • The cilia in the oviduct push the embryo toward the uterus the first cleavages occur along this journey. c) Rotational cleavage • First cleavage is meridional division. • In Second cleavage one of the two blastomeres divides meridionallt and other divides equatorially called rotational cleavage.
- 11. d) Asynchoronous celldivision • Mammalian blastomeres do not all divide at the same time. • Mammalian embryos do not increase exponentially from 2- to 4- to 8-cell stages. e) Early activation of the genome • The mammalian genome is activated during early cleavage, and produces the proteins necessary for cleavage to occur. • Example: In the mouse and goat, the switch from maternal to zygotic control occurs at the 2-cell stage.
- 12. Stages of developmentof mammalian embryo • Most research on mammalian development has focused on the mouse embryo. • Mice are relatively easy to bread • Can be housed easily in laboratories
- 14. Compaction • Mouse blastomerethrough 8 cell stage form a loose arrangement with plenty of space between them. • After the third cleavage, cell adhesion proteins like E-cadherin become expressed and blastomeres suddenly clump together and form a compact ball of cells this phenomenon is called compaction.
- 16. • This tightlypacked arrangement is stabilized by tight junctions that form between the outside cells of the ball, sealing off the inside of the sphere. • The cells within the sphere form gap junctions, thereby enabling small molecules and ions to pass between them.
- 18. Morula • The cellsof the compacted 8-cell embryo divide to produce a 16-cell morula (consist of small group of internal cells). a) Inner cell mass (ICM) • The internal cells are called inner cell mass. • They give rise to the embryo. b) Trophoblast • Most of the descendants of the external cells become the trophoblast (trophectoderm) cells. They produced no embryo. Form the tissue of chorion. • The formation of the trophoectoderm is the first differentiation event in mammalian development.
- 20. c) Totipotent The earliestblastomere can form both trophoblast and the embryo precursors cell these very early cells called totipotent. d) Pluripotent The inner cell mass is said to be pluripotent . Each cell of ICM can generate any cell type in the body.
- 21. e) Cavitation andblastocyst formation • Initially, the morula does not have an internal cavity. However, during a process called cavitation. • The trophoblast cells secrete fluid into the morula to create a blastocoel. • The plasma membranes of the trophoblast cells contain a sodium pump (a Na+/K+-ATPase) facing the blastocoel, and these proteins pump sodium ions into the central cavity. This accumulation of sodium ions draws in water osmotically, thus enlarging the blastocoels. • The inner cell mass is positioned on one side of the ring of trophoblast cells • The resulting structure, called the blastocyst, is another hallmark of mammalian cleavage.
- 24. Escape from thezonapellucida • While the embryo is moving through the oviduct en route to the uterus, the blastocyst expands within the zona pellucid. • The zona pellucida prevents the blastocyst from adhering to the oviduct walls. When such adherence does take place in humans, it is called an ectopic or tubal pregnancy. • A trypsin like protease secreted by the trophoblast seems responsible for hatching the blastocyst from the zona.
- 26. Implantation • Once out,the blastocyst can make direct contact with the uterus. The uterine epithelium (endometrium) “catches” the blastocyst on an extracellular matrix containing collagen, laminin, fibronectin, hyaluronic acid, and heparan sulfate receptors. 1. Loose attachment L_selectin on the trophoblast cell adhering to sulfated polysaccharides on the utrine cells.
- 27. 2. Stable attachement •The trophoblast cells synthesize integrinns on the trophoblast and uterine collagen, fibronectin and laminin and they synthesize heparin sulfate proteoglycan precisely prior to implantation. • p-cadherins on the trophoblast and utrine endometrium also help to attach the embryo to the uterus.
- 28. 3. Digestion ofuterine tissue • Once in contact with the endometrium, the trophoblast secretes another set of proteases, including collagenase, stromelysin, and plasminogen activator. These protein-digesting enzymes digest the extracellular matrix of the uterine tissue, enabling the blastocyst to bury itself within the uterine wall
- 30. Formation of ExtraembryonicMembranes 1. cytotrolphoblast 2. synctiotrophoblast 3. blood vessels and umbilical cord 4. chorion and decidusa-placenta formation
- 31. CYTOTROPHOBLAST • A layerof trophoblast cells • Adhers to endometrium (initially) • ContainProteolytic Enzymes remodeling of utrine Blood vessels • Screate Paracrine factors attract maternal blood vessels gradually displace vascular tissues (become lined with trophoblast cells
- 33. Synctiotrophoblast • Trophoblast cellsundergo nuclear division without cytokinesis • Multinucleated cells form synctiotrophoblast • help in progression of embryo into utrine wall (by digesting utrine tissue)
- 34. BLOOD VESSELS &UMBILICAL CORD • Mesodermal tissues extend outward from embryo • Yolk Sac + Primitive Streak derived cells form Extraembryonic mesoderm • Extraembryonic mesoderm joins the trophoblastic extensions give rise to blood vessels carry nutrients from mother to embryo • Narrow connecting stalk forms the vessels of umbilical cord
- 36. CHORION & DECIDUA(placenta formation) Fully developed extraembryonic organ trophoblast tissue + blood vessels containing mesoderm Fuses with decidua to creat placenta placenta Maternal portion (decidua) Fetal portion (chorion)
- 38. Embryo isencased in amnion & Chorion blood vessels to and from chorion are observable • blood vessels increased large area exposed to mother blood villi are projected on outer surface of chorion • diffusion of soluble substances occur through villi mother→nutrients & oxygen fetus→ wastes (CO2 & urea)
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