* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Embryology 2nd Stage Dr. Sameer Ahmed *Fertilization : Is the union of gametes (ovum and sperm) to from a single-celled zygote (fertilized egg) which eventually leads to the development of an embryo. *A series of dynamic and complex events are triggered following sperm– oocyte interaction that sequentially leads to fertilization and the formation of a zygote. Fertilization Events Sperm–Ova Association Fertilization begins with gamete fusion (zygote formation). The fusion of a spermatozoa with a secondary oocyte takes place in the uterine tube, near the ovary. **At the beginning, a spermatozoa binds to a specific glycoprotein on the zona pellucida that surrounds the oocyte ((this recognition process precludes union with foreign sperm)). **The spermatozoa releases degradative enzymes (acrosomal reaction) that allows the sperm cell to penetrate the zona pellucida. Acrosomal Reaction 1) The acrosomal reaction normally takes place in the ampulla of the fallopian tube (site of fertilization in human). 2) The sperm cell acquires a "hyperactive motility pattern" by which its flagellum produces vigorous whip-like movements that propel the sperm through the cervical canal and uterine cavity. 3) Glycoproteins on the outer surface of the sperm then bind with glycoproteins on the zona pellucida of the ovum. 4) The first stage is the penetration of corona radiata, by releasing hyaluronidase from the acrosome to digest cumulus cells surrounding the oocyte. 5) After reaching the zona pellucida the actual acrosome reaction begins. Acrosin digests the zona pellucida. 6) Part of the sperm cell membrane then fuses with the ova cell membrane, and the contents of the head sink into the ova. **Spermatozoa and oocyte plasma membranes fuse to form a zygote (the secondary oocyte completes meiosis). **After fertilization the oocyte precludes fusion with other sperm the zona pellucida undergo denaturing via enzymes released by exocytosis from oocyte cytoplasmic granules. **Male & female haploid pronuclei make contact, lose their nuclear membranes, and begin mitosis (mitosis begins 12 hours after sperm fusion; DNA synthesis takes place before mitosis) *Cleavage: Increasing the cell number by cell divisions (mitosis) during the early stage of embryogenesis immediately after fertilization. Each daughter cell of the cleavage process is termed a blastomere. Blastomere: is a type of cell which produced by cleavage of the zygote after fertilization. Thereafter, and due to the continuous development the blastomeres differentiate into inner & outer cells masses. Stages of Cleavage Morula Stage (small mulberry): Is a solid ball of blastomeres, within a zona pellucida. A morula typically consists of 16 to 64 blastomeres = four to six cell divisions. Blastomeres become compacted cells packed on the inside differentiate from those along the surface of the morula: — outer blastomeres become flattened and form tight junctions (resulting in reduced permeability to fluids); they develop the capacity to secrete fluid (internally); they are destined to become trophoblasts which form the chorion & amnion (fetal membranes) ::: Trophoblasts: the cells distributing around cavity of a blastcyst — inner blastomeres form gap junctions to maximize intercellular communication; they are destined to become inner cell mass (embryo blast) which forms the embryo (plus two fetal membranes). Blastula Stage (Blastocyst): Blastula, a ball of cells with a central cavity, one of the result of development the embryo is an accumulation of fluid inside the central cavity, which signals to formation of the Blastocyst or Blastula Stage. The fluid is called blastocoele (blastocyst cavity). Gastrula Stage or Gastrulation: Gastrulation or germ layer formation is a stage of embryological development during which the single layered blastula is converted in to trilaminar structure (trilaminar embryonic disc) that gives rise to three germ layers: Ectoderm, Mesoderm, and Endoderm. (In a gastrula [Gr.= little stomach] one can see evidence of primitive gut formation.) which ultimately give rise to specific tissues and organs. Gastrulation events includes the following sequence, beginning with a blastocyst: — A thickened embryonic disc becomes evident at the blastocyst surface, due to cell proliferation of the inner cell mass cells. Trophoblast cells overlaying the inner cell mass degenerate in domestic mammals (in some mammals, e.g., mouse and human, trophoblast cells overlaying the inner cell mass separate and, instead of degenerating, become amnionic wall.) — From the inner cell mass, cells proliferate, break loose (delaminate), and migrate to form a new cell layer inside the trophoblast layer. The new layer of cells is called the hypoblast (endoderm) ; it forms a yolk sac. The remaining inner cell mass may henceforth be called epiblast (ectoderm). — On the epiblast surface, a primitive streak forms as differential cell growth generates a pair of ridges separated by a depression. [NOTE: The primitive streak defines the longitudinal axis of the embryo and indicates the start of germ layer formation.] — The separation of the hypoblast layer from the epiblast establishes a space (coelom/celom) deep to the primitive streak. Subsequently, the coelom is temporarily filled by mesoderm that undergoes cavitation to restablish the coelom that gives rise to body cavities. — Epiblast cell proliferation along primitive streak ridges becomes the source of a cellular migration through the streak depression. The migrating cells form endoderm & mesoderm layers. The Germinal Layers An adult, multicellular animal typically possesses a concentric arrangement of tissues of the body. These adult tissues are derived from three embryonic cell layers called germinal layers; the outer layer is the Ectoderm, the middle layer is the Mesoderm, and the innermost layer is the Endoderm.