Chapter 17, Problem 1RQ

Define and describe the main features of the following developmental stages: fertilization, cleavage, gastrulation.

Summary Introduction

To define: The main features of the following developmental stages: fertilization, cleavage, and gastrulation and describe it.

Introduction: Gestation or pregnancy is the period during which an offspring develops within a woman. This involves several developmental stages of fetus. In humans, pregnancy is nearly nine months long, divided into three trimesters, involving several changes in the mother and the growing fetus.

Explanation of Solution

Fertilization

Fertilization is a phenomenon where the two haploid gametes (male gamete-sperm and the female gamete-egg) unite to form a new diploid cell zygote that consists of genetic materials derived from both parents.

Events of fertilization:

1. 1. Corona radiata penetration: Corona radiata and zona pellucida are the two layers in the secondary oocyte that initially prevents the entry of a sperm. Once the sperm reaches the corona radiata, their motility allows them to push through the cell layer and penetrate the corona radiata to reach zona pellucida.
2. 2. Zona pellucida penetration: After penetrating the corona radiata, the acrosomal structure of sperm releases digestive enzymes (hyaluronidase and acrosin) to penetrate the zona pellucida layer. At that time, the sperm enters the nucleus of the secondary oocyte, immediately certain changes occur to the zona pellucida layer and the oocyte to ensure that no other sperm can enter the oocyte again. For ensuring that only one sperm fertilizes the oocyte, the zona pellucida layer hardens to prevent the binding of others sperms to it.
3. 3. Fusion of sperm and oocyte plasma membrane: Fusion of sperm and oocyte occurs immediately when they come into contact. Only the nucleus of the sperm enters the oocyte, whereas the midpart and flagellum degenerate shortly. A mature ovum is formed as the sperm nucleus enters the secondary oocyte and secondary oocyte completes the secondary meiotic division. The nucleus of the sperm and ovum has a haploid number of chromosomes and are termed as pronuclei. A single diploid cell called zygote is formed when these haploid pronuclei come together and fuse to form a single cell.

Cleavage

The zygote is formed by a fusion of an egg cell and a sperm cell. The zygote immediately after fertilization starts dividing. After the two-celled stage (after primary division), zygote undergoes a series of mitotic divisions. The division that results in an increase in cell number but not an increase in the overall size of the structure is termed as cleavage. Greater numbers of smaller cells are produced by the mitotic divisions to fit the cells in the overall structure. The diameter of the structure remains about 120 µm and will not increase in the size until the structure implants in the wall of the uterus and derives a nourishment source from its mother.

Gastrulation

Gastrulation is an early phase that occurs during the third week of embryonic development. During this process, the cells of the epiblast migrate to form three primary germ layers that later give rise to different body parts. The three germ layers are: outer ectoderm, middle mesoderm, and inner endoderm. The embryo (trilaminar structure) is developed after the formation of these three layers. All three layers give rise to different body parts.

Tabular representation: The following table shows the three primary germ layers that compose the embryo

Table 1: Three primary germ layers

Three primary germ layers	Description
Ectoderm	It forms nervous system, epidermis, sense organs, most exocrine glands, some endocrine glands, tooth enamel, and lens.
Mesoderm	It forms most muscles, connective tissues, cardiovascular system, urinary system, and reproductive system.
Endoderm	It forms an inner lining of the respiratory, digestive, urinary, and reproductive tracts. It also lines the portion of the liver, pancreas, palatine tonsils, thyroid, parathyroid, and thymus gland.