BIOLOGY Lecture Notes - Golden Pages 2003

5. BASICS OF EMBRYOLOGY

A new Organism arises As a result of the fusion of the genetic material of two Gametes: a sperm and an egg. This fusion is called Fertilization, which results in The Development of the egg. The sequence of stages following fertilization is known as Embryogenesis.

Stages of embryogenesis represent various modifications of four main types:

1. Cleavage occurs immediately after fertilization. Cleavage is a series of rapidly occurring mitotic divisions, as a result of which the volume of the zygote's Cytoplasm is divided into numerous smaller Cells. These cells are called blastomeres, and by the end of cleavage, they usually form a ball known as a blastula.

2. Following a decrease in The rate of mitotic divisions, the blastomeres undergo vigorous movements, resulting in A change in their relative positions. This series of Cell movements is called Gastrulation. As a result of gastrulation, the embryo typically forms III cell layers, known as germ layers. The outer layer, the ectoderm, gives rise to the epidermis and The Nervous system; the inner layer, the endoderm, forms the lining of the digestive tract and participates in The formation of associated Organs (the Pancreas, Liver, etc.) and respiratory organs; the middle layer, the mesoderm, gives rise to several organs (Heart, Kidneys, Gonads), Connective Tissue (bones, tendons, and formed elements of the Blood), and Muscles.

3. After the appearance of the three germ layers, the cells interact with one another and migrate to form organs. This process is called Organogenesis. Many organs contain cells originating from different germ layers; frequently, the outer part of an organ is formed by cells of one germ layer, and the inner part by another. During organogenesis, some cells undertake long-range Migrations from their site of origin to their final destination. Such migrating cells include precursors of BLOOD AND Lymph elements, pigment cells, and gametes.

4. Part of the egg cytoplasm gives rise to precursor Cells of the gametes. These cells are called primordial Germ Cells, and they segregate to perform the reproductive function. All other cells are called somatic. This division into somatic cells (which give rise to the individual organism) and germ cells generally represents one of the first stages of differentiation.

Primordial germ cells subsequently migrate into the gonads, where they differentiate into gametes—cells involved in fertilization. The development of gametes, called gametogenesis, is usually completed by the time the organism reaches sexual maturity.

Cleavage: Establishment of Multicellularity

Cleavage is a series of mitotic divisions that subdivide the volume of the egg cytoplasm into numerous smaller, nucleated cells. These cells, formed during the cleavage period, are called blastomeres. During cleavage, the size of the embryo does not increase, and the blastomeres become smaller with each division. The energy for the cleavage process is supplied by The breakdown of Glycogen and other substances stored in the cytoplasm. The total mass of the blastula (its dry mass) may even decrease compared to the mass of the unfertilized egg.

Cleavage is the result of coordinated processes. The first of these is karyokinesis (mitotic nuclear division). The mechanical apparatus for this division is the mitotic spindle with its cytoskeletal structures, the microtubules. The second process is cytokinesis, or Cell Division. The mechanical apparatus of cytokinesis is the contractile ring of microfilaments.

Class="center">Rotational Holoblastic Cleavage

Mammalian eggs are among the smallest in the animal kingdom. The diameter of a human zygote, for instance, is 100 µm.

Upon release from the Ovary, the mammalian oocyte enters the oviduct. Fertilization takes place in the ampulla of the oviduct, a region located close to the ovary. At this time, Meiosis is completed, and the first cleavage division begins approximately a day later.

Mammalian cleavage is among the slowest. Each division takes from 12 to 24 hours. A distinctive feature of cleavage is the characteristic arrangement of blastomeres relative to one another. The First Division is a normal meridional division; however, during the Second Division, one of the two blastomeres divides meridionally and the other equatorially. This type of cleavage was termed rotational (Gulyas, 1975). The third important difference in mammalian cleavage is the marked Asymmetry (asynchrony) of early cleavage. Blastomeres do not all divide simultaneously, so embryos do not exhibit a uniform increase in blastomere number from the 2-cell to the 4- and 8-cell stages; embryos frequently contain an odd number of cells.

Compaction

The most important difference between mammalian cleavage and all Other types of cleavage is The phenomenon of compaction.

At the 8-cell stage, mammalian blastomeres are loosely arranged, with large spaces remaining between them. Following the third division, The behavior of the blastomeres changes. They suddenly draw closer together, the contact area between them increases, and they form a compact cell mass. This tight "packing" is stabilized by tight junctions between the cells located On the surface of the ball, isolating the internal cells. Gap Junctions form between the cells inside the ball, allowing small molecules and ions to pass from Cell to Cell.

The cells of the compacted embryo divide to form a 16-cell morula. Such a morula consists of a small number of internal cells surrounded by numerous external cells. Most of the external cells become trophoblast (trophectoderm) cells. This group of cells does not form embryonic structures, but instead develops into the chorion, which participates in the Formation of the Placenta. The placental Tissues provide the fetus with oxygen and nutrients from the mother; secrete Hormones; and synthesize Immune Response regulators that prevent the maternal organism from rejecting the embryo.

Trophoblast cells are essential for the implantation of the embryo into the uterine wall.

The embryo is formed by the descendants of the internal cells of the 16-cell embryo, sometimes augmented by a cell that separated from the trophoblast during the transition to the 32-cell stage. These cells form the inner cell mass (ICM), which gives rise to the embryo itself. ICM cells differ from trophoblast cells both in appearance and in THE SPECTRUM OF Proteins they synthesize at this early stage. By the 64-cell stage, the inner cell mass and trophoblast cells have transformed into fully differentiated cell layers, neither of which contributes cells to the other group. Thus, The Emergence of differences between trophoblast blastomeres and the inner cell mass is a process of differentiation in mammalian development.

Initially, the morula lacks an internal cavity. However, during cavitation, trophoblast cells secrete fluid into the morula, leading to the formation of the blastocyst cavity (The inner cell mass is located on one side of the hollow sphere formed by the trophoblast cells. This Structure is called the blastocyst.).

Thus, in mammals, a unique type of cleavage gives rise to the elements of the placental rudiment, which will supply the embryo with nutrients. In all cases, karyokinesis and cytokinesis must be coordinated, and the egg partitioned into distinct cellular domains. During cleavage, the nucleocytoplasmic ratio characteristic of somatic cells is restored, and developmental information is segregated among different cellular regions.