Introduction
Embryology is the study of amazing integrated complex phenomena in which a single cell (zygote) has been grown into a baby in 280 days. Today the scope of the embryology is much advanced. The investigations ultrasound, maternal serum screening, amniocentesis, cephalocentesis, obstetric intervention and chorionic villus sampling are to detect malformations, genetic abnormalities, fetal growth and the complications of pregnancy; these knowledge of embryology are useful to correct the malformations of the fetus in utero itself. The advancement of embryology is creating public awareness for better reproductive outcomes; the preventive measures are also growing along with the growth of the fetus. The knowledge of embryology is solving the social problems like the infertility and infant mortality in new techniques for in utero diagnoses and treatments.
Embryology speaks the stages of development of the embryo from the stage of fertilization to the birth of a child. The basic principles in development are (a) growth of cells and (b) differentiation of cells. After first cleavage of the zygote, each of the two cells can produce a separate embryo which is the totipotent cell. This is the basis for identical or uniovular twins. The totipotent cell becomes pleuripotent which gives specific type of tissue.
The cell differentiation by chemical stimulus is chemodifferentiation, the chemodifferentiation is succeeded by histodifferentiation in which different types of tissues are formed; then the differentiated cells are moulded into different organs.
Female Germ Cells
Primordial Germ Cells
The ovaries are paired reproductive gonads of the female. The primordial germ cells are with in the peripheral cortical area of the ovary (Fig. 1.1). These germ cells are migrated from the yolk sac of the embryo; in female the primordial germ cells are called Oogonia. The oogonia increase in size and differentiate into primary oocyte; when the primary oocyte of epithelial cells (follicular cells), the primary oocyte becomes primary ovarian follicle (Fig. 1.2).
Out of millions of primary ovarian follicle only 400–500 follicles reach the stage of secondary oocyte (nearing matured/female gamete/ovum) after puberty.
Oogenesis
Oogenesis is maturation of female primordial germ cells from primary Oocyte to ovum with only first meiosis (second meiosis occurs only at the time of fertilization) when ovarian follicle undergoes maturation, the primary oocyte becomes larger, the follicular cells becomes cuboidal forming several layers of cells.
The scattered amorphous materials between the oocyte and follicular layer becomes zona pellucida. Along with enlargement of follicle, fluid filled spaces appear, they coalesce and form antrum folliculi within the follicular cells, this results in formation of membrana granulosa (external to antrum folliculi) and in formation of cumulus ovaricus or cumulus oophorus (internal to antrum folliculi); but both stratum granulosum and cumulus ovaricus are continuous at one end.5
Outside the stratum granulosum, stromal cells are differentiated to form tunica interna, tunica externa, and basement membrane between the tunica interna and stratum granulosum; with these coverings the follicle becomes nearing matured as Graafian follicle (Fig. 1.3). The process of maturation of Graafian follicle started in utero itself, this process is completed by forming matured Graafian follicle (ovum) in the cortex of the ovary.
During this maturation processes of ovum, the primary oocyte undergoes first meiotic cell division; in the first meiotic division two daughter cells are formed with haploid chromosomes (22 autosomes + 1 sex chromosome), in this one pair only one comes to the central position and the other passes outwards to become first polar body. The cell which is having nucleus at the central becomes larger, receives abundant cytoplasm from the mother cell is known as secondary oocyte (ovum). This ovum (10 mm size) is the largest cell in human (in male largest cell is betz cell of cerebrum, but betz cell is smaller than ovum). The ovum is called female gamete. By above narration, oogenesis/maturation of ovum is first meiosis plus cytodifferentiation (haploid chromosomes) (Fig. 1.4).
Ovulation
Release of the ovum (matured Graafian follicle) (Fig. 1.5) from the surface of the ovary is called ovulation. Ovulation occurs at the middle of the menstrual cycle (28 days).6
Once ovum is released, if it is not fertilized, it is expelled after 14 days of its release along with endometrial bleeding called menstruation.
The ovum after it's release from the ovary, it comes to the fimbriated end (abdominal osteum of fallopian tube); further transportation of ovum to the ampullary part of tube is by cilia of the tubal epithelium and by peristaltic contraction of musculature of the tube. The ampullary part of fallopian tube is the site of union of spermatozoon with ovum (fertilization).
Male Germ Cells
Male gonad is testis, it is also paired situated within the scrotum. In male, primordial germ cell is spermatogonia (Fig. 1.6).
In the seminiferous tubules of the testes, specialized supporting cells called cells of Sertoli, the function of the Sertoli cells, (1) provide nutrition for developing germ cells and (2) provide antigen free environment for spermatogenesis and phagocytose the waste from the spermatids.
Spermatogenesis
Spermatogenesis is a series of stages by which the primordial germ cells passing into spermatozoa. Spermatogenesis consists of three stages, spermatocytosis, meiosis and spermiogenesis.
By spermatocytosis spermatogonia undergoes mitotic cell divisions, at the end of this stage, primary spermatocytes are formed with 44 + XY chromosomes (Fig. 1.7).
Meiosis
The primary spermatocytes undergoes first meiotic division, each gives two varieties of cells with chromosomal reduction (haploid); one is with 22X chromosomes, the other with 22Y chromosomes. After the first meiotic division unlike oocyte, the primary spermatocyte gives two equal secondary spermatocytes.8
The secondary spermatocytes further undergoes second meiotic division and give spermatids. Out of one primary spermatocyte four equal spermatids are formed whereas in the female one primary oocyte gives only one ovum with three polar bodies. Among the four spermatids, two are X-bearer and other two are Y-bearer.
Spermiogenesis
Spermiogenesis is transformation of spermatid into spermatozoa.
The following are the morphological changes (1) acrosomal granules of Golgi apparatus are forming a covering over the nucleus called head cap, (2) the centrosome splits to form axial filaments of the body and tail of the spermatozoa, (3) the mitochondria form sheath for the body of the spermatozoa and (4) most of the cytoplasm will be lost. After acquiring the maturity by these stages the matured spermatozoa freed and found with in the lumen of the seminiferous tubules (Fig. 1.8).