What is Embryology ? Embryology | Anjani Mishra

 Written By Anjani Mishra

What is animal embryology ?

Animal Embryology is the study of the growth and differentiation undergone by an organism in the course of its development from a single fertilized egg cell into a highly complex and independent living being like parents.

Branches/fields in Embryology

• Descriptive Embryology: Description of the structural features of embryos of various ages.
• Comparative Embryology: Compare among the embryos of different animals.
• Experimental Embryology: Genetics/ Cloning/Test tube.
• Chemical Embryology: Studies regarding chemical components of embryo. for e.g.; DNA.
• Teratological Embryology: Study of malformation. for e.g. monster baby.

Objective

• Gain the knowledge of evolution.
• Know the teratological defects.
• Apply the knowledge towards pathology, immunology and other applied subjects.
• To increase the conception rate of animals.
• Commercial purpose – Cloning of animals and its marketing.

Methods of study   

• Dissection – Gross dissection of embryos at different ages of animals.
• Microscopic Section – At different ages of animals.
• Descriptive Study – Sections, photography.
• Experimental Study ­­- By various drugs/ chemicals/ physical measurements.

Gametogenesis

• Gametogenesis is the process of maturation of primordial germ cell into mature gametes (i.e. male and female gametes) which are specialized sex cells that contain haploid number of chromosomes.
• Simply, gametogenesis is the process by which the sex cells or gametes are produced.
• Gametogenesis occurs both in male and female.
• In male it is known as spermatogenesis where as in case of female it is known as oogenesis.
•  Generally three phases of development are noticed in these process of gametogenesis.

Spermatogenesis

• Spermatogenesis is a process of maturation of primordial male germ cells into mature spermatozoa.
• Spermatogenesis consists of three stages of development, such as;

1. Spermatocytosis

2. Meiosis

3. Spermiogenesis

Fig: Ultramicroscopic structure of a seminiferous tubule of testis

1. Spermatocytosis

• It is the process of maturation of primitive germ cells into primary spermatocytes. In case of male, the formation of primitive or primordial germ cells are completed before birth and the differentiation of primordial germ cells begins at puberty, however in female it is completed before birth.
• It begins in uterus during 3^rd month of development.
• At the time of birth, germ cells in the male can be recognized in the sex cords of the testis as large, pale cells surrounded by supporting cells.
• At about the same time primordial germ cell give rise to spermatogonia, which consists of two types of cells, such as;

Type A- Which divide by mitosis to provide continuous reserve of stem cells.

Type B- Which gives rise to primary spermatocytes.

Fig: Seminiferous tubules of testis of dog

3. Lumen,  4. Myoid cell,  5. Primary spematocyte,  8. Sertoli cell nucleus,  10. Spermatid early,  11. Spermatid late,  12. Spermatogonia

2. Meiosis

• After formation of primary spermatocytes, they enter the 1^st maturation division as meiosis-I where secondary spermatocytes are formed that contain haploid number of chromosomes.
• These cells differentiate immediately to form spermatid during the 2^nd maturation division as meiosis-II, which contain the haploid number of chromosomes.
• These cells remain embedded in sertoli cells throughout their development that provide support and protection.

 Fig: Seminiferous tubule of testis of horse

2. Interstitial cell,  4. Myoid cell nucleus,  5. Primary spematocyte,  8. Sertoli cell nucleus,  
10. Spermatid early,  11. Spermatid late,  12. Spermatogonium,  13. Straight tubule

3. Spermiogenesis

• It is a process of maturation of morphological transformation of spermatid into spermatozoa without further cell division.
• Simply, the series of changes resulting in the transformation of spermatid into spermatozoa.
• During this process, following developments or changes takes place;
• These changes include:

a) Formation of acrosome, which contains enzymes to assists in penetration of ovum and its      surrounding layers during fertilization.

b) Formation of neck, middle piece and tail.

c) Shedding of most of the cytoplasm.

d) Condensation of the nucleus.

e) In human the time required for spermatogenesis is approximately 64 days.

Oogenesis

• Oogenesis is the process of maturation of primordial female germ cells into mature ovum.
• Oogenesis consists of three phases of development, such as;

1.      Growth of sex cells

2.      First maturation division/ reduction division/ Qualitative division

3.      Second maturation division/ non-reduction division/ Quantitative division

Atretic follicle:

Degeneration of the ovarian follicle is known as atretia and the degenerating follicle is known as atretic follicle.

 

Number of chromosome – Ox – 60                     Dog – 78

                                           Horse – 64                Fowl – 78                    

                                           Pig – 38                      

1.  Growth of sex cells

    Prenatal maturation:

• The female primordial germ cells in the gonad starts to differentiate into oogonia.
• These cells undergo a number of mitotic cell division and form primary oocyte.
• These cells by the end of 3^rd  month arranged in cluster and are surrounded by a layer of flat epithelial cells.
• These flat epithelial cells originate from surface epithelial covering of the ovary is known as follicular cells.
• These follicular cells together with primary oocyte is known as primordial follicle. 

Fig: Ovary of queen

11. Primary follicle,  12. Primordial follicle,  13. Stroma

2.  First maturation division/ reduction division/ Qualitative division

     Post-natal maturation:                   

• The primordial follicle begin to mature with each estrus cycle with the onset of puberty.
• The primary oocyte begins to increase in size, where as the surrounding flat follicular cells change from flat to cuboidal and further proliferate to produce stratified epithelium as granulosa cells.
• These cells are known as primary follicle.
• These primary follicle develops to form secondary follicle, where small spaces are found in the follicular mass called antrum.
• Antrum is filled with fluid called liquor folliculi.
• During this period, cells of theca folliculi becomes organized into inner layer of secretory cells called theca interna and outer layer of connective tissue containing fibroblast like cells called theca externa.
• The secondary follicle further develops to form graafian follicle, where several changes takes place, such as; formation of cumulus oophorus, corona radiata, membrana granulosa, expansion of antrum etc.
• These follicle contains haploid number of chromosomes.

 Fig: Ovary of bitch

1. Antrum,  4. Granulosa lutein cell,  7. Membrana granulosa,  8. Theca externa,  

9. Theca interna

3.  Second maturation division/ non-reduction division/ Quantitative division

• The second maturation division is completed only if the oocyte is fertilized, otherwise the cells degenerate approximately 24 hours after ovulation.
• After fertilization, the secondary oocyte form mature ovum containing most cytoplasm and other remain as 2^nd polar body, which ultimately degenerate.
• Mature ovum contains haploid number of chromosomes.

 
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