Phases of embryonic development.
Following are the five phases of embryonic development of sexually reproducing metazoans
i. Gametogenesis It is the production of gametes. It includes spermatogenesis (formation of sperms) and oogenesis (formation of ova).
ii. Fertilization It is the process whereby two gametes fuse together to create a new individual with a genome derived from both parents.
iii. Gestation It is the time from conception to birth. In human being, it is approximately 270 days. Three sequential processes occur during gestation (pregnancy).
iv. Parturition The process of birth is known as parturition.
Cleavage and Blastulation
Cleavage includes a series of successive and rapid mitotic divisions, which transform single-celled zygote into a multicellular body consisting of many cells called blastomeres. The cleavage ends with the formation of a blastula. Formation of the blastula is called blastulation.
Gastrulation Formation of gastrula is called gastrulation. Gastrula is a reorganized form of blastula. The gastrula consists of germ layers. The latter give rise to complete individual.
Organogenesis is a process by which different organs and organ-systems are formed from the germ layers. In fact the cells of the germ layers give rise to the tissues, organs and organ systems.
Production of gametes
Gametogenesis is the process by which male and female sex cells or gametes, i.e. sperms and ova are formed respectively in the male and female gonads (testes and ovaries). Meiosis is the most significant part of process of gametogenesis. Gametogenesis for the formation of sperms is termed spermatogenesis, while that of ovum is called oogenesis. Both spermatogenesis and oogenesis comprise similar phases of sequential changes viz,
i. Multiplication phase
ii. Growth phase
iii. Maturation phase
The place of occurrence of spermatogenesis is the seminiferous tubules of the testes, which are lined by germinal epithelium, which consists largely of cuboidal primary or primordial germ cells (PGCs) and contains certain tall somatic cells called Sertoli cells or the nurse cells. Spermatogenesis includes formation of spermatids and formation of spermatozoa.
Formation of spermatids includes the following phases:
i. Multiplication phase At sexual maturity, the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia. Spermatogonia (2N) are of two types type A spermatogonia and type Bspermatogonia. Type A spermatogonia serve as stem cells which divide to form type A spermatogonia and type B spermatogonia. Type B spermatogonia are the precursors of sperms.
ii. Growth phase Each type B spermatogonium actively grows to a larger primary spermatocyte by obtaining nourishment from the nursing cells.
iii. Maturation phase Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional division or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form, four haploid spermatids, ultimately form four from each primary spermatocyte.
iv. Formation of spermatozoa from spermatids (spermiogenesis) The transformation of spermatids into spermatozoa is called spermiogenesis or spermateliosis. The spermatozoa are later on known as sperms. Thus four sperms are formed from one spermatogonium.
Hormonal control of spermatogenesis Spermatogenesis occurs under the influence of hormones. Gonadotropin releasing hormone (GnRH) is secreted by the hypothalamus. It stimulates the anterior lobe of the pituitary gland to secrete LH and FSH. In male, LH is called interstitial cells stimulating hormone (ICSH) because it stimulates interstitial cells (Leydig‘s cells) of the testes to secrete androgens. Testosterone is the principal androgen. FSH stimulates sertoli cells of the testes to secrete an androgen-binding protein (ABP) that concentrates testosterone in the seminiferous tubules. Sertoli cells also secrete a protein hormone called inhibin which suppresses FSH synthesis. FSH acts directly on spermatogonia to stimulate sperm production.
It occurs in the ovaries (female gonads). It consists of three phase multiplication, growth and maturation.
i. Multiplication phase: In the foetal development, certain cells in the germinal epithelium of the ovary are larger than others. These cells divide by mitosis, producing undifferentiated germ cells called egg mother cells or oogonia in the ovary of the foetus. The oogonia multiply by mitotic divisions and project into the stroma of the ovary as a cord, the egg tube of Pfluger which later becomes a round mass, the egg nest. One cell in the egg nest grows and becomes the primary oocyte. The later is the future ovum. Other oogonia in the nest form the follicular epithelium, round the primary oocyte to protect and nourish it. The structure, thus formed, is called ovarian (Graafian follicle).
ii. Growth phase: This phase of the primary oocyte is very long. It may extend over many years. The oogonium grows into a large primary oocyte by taking food from the surrounding follicle cells. It happens after puberty.
iii. Maturation phase Like a primary spermatocyte, each primary oocyte is also fated to undergo two maturation divisions, first meiotic and the second meiotic. The results of maturation divisions in oogenesis are, however, very different from those in spermatogenesis. In the first, meiotic division, the primary oocyte divides into two very unequal haploid daughter cells; a large secondary oocyte and a very small first polar body or polocyte.
The process in which the two sex cells, called gametes, fuse together to create a new individual with genetic potentials derived from both parents is known as fertilization. Two distinct ends have been accomplished by the process called sex which is the combining of the genes that are derived from two parents, and reproduction (i.e. creation of new individual).
Hence, the function of fertilization is to transmit the genes from parent to offspring and to initiate the reactions in the egg cytoplasm to permit the proceedings of the development.