Cleavage and Implantation Animal Development

Embryogenesis in animals is started by fertilization of the egg by a male sperm in the fallopian tubes (oviduct), which is followed by Cleavage 24 hours after fertilization. The single-celled diploid zygote formed undergoes mitotic cell division, which increases the number of cells and not the volume. The single-celled zygote is cleaved into a multicellular embryo (blastocyst). Cleavage is of two types based upon the yolk egg concentration: (i) holoblastic where complete cleavage of the cell takes place and (ii) meroblastic in which cleavage is incomplete. Cleavage is followed by gastrulation in which three germ cell lines are formed which further divide to form organs and nervous systems of the cell. The multicellular embryo, now moving as blastocyst finally enters the uterus, where it gets implanted into the endometrium epithelial lining of the uterus for maternal support is known as implantation.

Types of Cleavage
The yolk is a deposit of food in the egg that is used as energy by the cell for growth and development. Based on the concentration and distribution of yolk, the cell is divided into two hemispheres. i. the Animal pole with low yolk concentration and is mostly filled with the cytoplasmic space where rapid cell division takes place ii. The vegetable pole is the one where large yolk cells are present in which slow cell division is observed. Based upon the yolk concentration the cleavage is classified as: 1. Holoblastic: Mostly occurs in amphibians and mammals, where a moderate or sparse amount of yolk is present and complete cleavage takes place. The cell is cleaved from the animal pole to the vegetable pole, each cleavage is perpendicular to the other. Based upon the distribution of the yolk, holoblastic based on the yolk distribution is further divided into

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Isolecithal – uniform distribution of yolk is observed in the cells. eg: Amphioxus, invertebrates, and mammals

Based on the cleavage pattern isolecithal are further divided into- bilateral, radial, rotational and spiral

Mesolecithal- moderate distribution of yolk is observed in the cells. eg: cartilaginous, fish, reptiles, birds, and monotreme mammals. 2. Meroblastic: This type of cleavage is observed in cells where the density of the yolk concentration is more so that it gets accumulated in the vegetable pole of the cell. The cleavage furrow could only cleave the animal pole and is impermeable through the vegetable pole leaving an incomplete cleavage. Meroblastic cleavage, based upon their distribution of yolk is further divided into

Telolecithal- The yolk is present in one end of the cell, mostly in the vegetable pole thus, the cleavage furrow is unable to pass through the vegetable pole due to its increased yolk concentration.

Cleavage occurs only in the animal pole in which the cytoplasmic disc is present. This type of cleavage is known as discoidal Eg; crustaceans, cephalopod, gastropod mollusks, vertebrates, including fish, amphibians, reptiles, birds, and monotremes.

Centrolethical- The yolk is present in the center of the cell, thus cleavage occurs in the peripheral of the egg known as superficial cleavage eg: arthropods and some coelenterates.

Implantation in Animal Development
During implantation, the blastocyst enters and implants itself into the uterine wall and remains there till birth for maternal support. Once the blastocyst enters the uterus, degradation of zona pellucida (a protective layer) around the blastocyst takes place. Adhesion and opposition between the trophoblast (outer layer of blastocyst) and the endometrium uterine epithelium decide the level of implantation. Preimplantation starts with the adhesion of the microvilli in the endometrial epithelial cell lining of the uterus to the trophoblast of the blastocyst, which makes it implantation competent. However, organism type also plays a major role in implantation. Example: eutherian mammals develop a placenta which helps it to connect to the uterus, while marsupials get loosely implanted in the uterus and early birth takes place, while maturation takes place in the maternal pouch (marsupial). In some oviparous, the egg is directly deposited in the maternal pouch from the cloaca for incubation and maturation. Role of biochemicals in implantation:

Estrogenic causes endometrium proliferation, thickening, and expansion to create a hostile environment for the growth of the embryo

Endometrium decidualization is caused by the hormone progesterone.

Integrins extend the uterus lining and cause cell adhesion, modulation in the local immune system, and gene expression. Types of implantation: 1. Centric implantation: Blastocytes increases their size and fuses into the luminal epithelium of the uterus and implants themselves in the center of the uterus eg: carnivores and domestic animals. 2. Eccentric implantation: Blastocysts are small in size thus the endometrium surrounds the trophoblast layer by an invagination formation. Eg: rat, mice 3. Interstitial: Blastocyst erodes the endometrial lining to reach the subepithelial lining and gets embedded into the uterus walls eg: primates including humans and guinea pigs.

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