Introductions
* Embryology deals with the development of an organism frog the egg to an adult.
* The word embryo refers to the juvenile stages of an animal, when it is inside the egg or inside the maternal body.
* A young onw when it has first come out of the egg or just born cannot be called an embryo.
* The development of an animal starts from gametogenesis and proceed with fertilization, cleavage, gastrulation, organogenesis, growth, differentiation, metomorphosis regeneration etc.
Gametogenesis
* The production of gametes is called gametogenesis.
* It occurs in the gonads.
* Gametes are of two types, namely spermatogenesis and egg.
* The productions of spermatozoon is called spermatogenesis.
* It takes place in the testis.
* The production of egg (ovum) is called oogenesis.
* It occurs in the ovary.
Primordial germ cells :
* The cells which are destined to develop into gametes are called primordial germ cells.
* The gametes and the primordial germ cells constitute the germplasm.
* The germplasm is of paramount importance because the somatoplasm cease to exist with the death of animals.
* The germplasm continues to live o indefinitely in succeeding generations.
* The germplasm of an individual has come from its parents by way of the ovum and the sperm.
* The future of the human race depend on the germplasm held in trust within the bodies of the individuals now living.
The gametogenesis
* Is the process of gamete formation in the sexually reproducing animals.
* The sexually reproducing animals contain two types of cells in their body, e.g., somatic cells and the germinal cells.
* Both types of cells have diploid number of chromosomes but each type has its different destiny.
* The somatic cells from various organs of the body.
* The somatic cells always multiply by mitotic division.
* The germinal cells from the gonads (testes and ovaries) in the animal body.
* These cells produce the gamete cells by successive mitotic and meiotic divisions.
* The male gamete is known as spermatozoon or sperm and the female gamete is known as ovum or egg.
* The process or sperm production is known as the spermatogenesis (Gr., sperma=sperm or seed; genesis=origin).
* The process of production of ovum is known as oogenesis (Gr., oon=egg)
*Both the processes can be studied in detail under separate headings.
Spermatogenesis
* The process of spermatogenesis occurs in the male gonads or testes.
* The testes of the vertebrates are composed of many seminiferous tubules which are lined by the cells of germinal epithelium.
* The cells of the germinal epithelium form sperms by the process of spermatogenesis.
* But in certain animals, e.g., mammals and Mollusca, etc., there are somatic cells lying in between germinal cells.
* These somatic cells are known as sertoli cells.
* The sertoli cell anchor the differentiating cells and provide nourishment to the developing sperms.
* The insects do not posses sertoli cells.
* The spermatogenesis is a continuous process and for the sake of convenience this process can be studied in two different stages.
1. Formation of spermatids;
2. Spermiogenesis
Formation of spermatids :
* The male germinal cells which produce the sperms are known as the primary germinal cells or primordial cells.
* The primordial cells pass through following three phases for the formation of spermatids:
i) Multiplication phase
ii) Growth phase
iii) Maturation phase
i) Multiplication phase :
* The undifferentiated germ cells or primordial cells contain large-sized and chromatinrich nuclei.
* These cells multiply by repeated mitotic divisions and produce the cells which are known as the spermatogonia (Gr., sperma = sperm or seed; gone = offspring).
* Each spermatogonium is diploid and contains 2X number of chromosomes.
ii) The growth phase :
* In the growth phase, the spermatogonial cells accumulate large amount of nutrition and chromatin material.
* Now each spermatogonial cell is known as the primary spermatocyte.
iii) The maturation phase :
* The primary spermatocytes are ready for first meiotic or maturation division.
* The homologous chromosomes start pairing (synapsis), each homologous chromosomes splits longitudinally and by the chiasma formation the exchange of genetic material or crossing over takes place between the chromatids of homologous chromosomes.
* The DNA amount is duplicated in the beginning of the division.
* By first meiotic division or homotypic division two secondary spermatocytes are formed.
* Each secondary spermatocyte is haploid and contains X number of chromosomes.
* Each secondary spermatocyte passes through the second maturation or second meiotic or heterotypic division and produces two spermatids.
* Thus, by a meiotic diploid spermatogonium produces four haploid spermatids.
* These spermatids cannot act directly as the gametes so they have to pass through the next phase, the spermiogenesis.
Spermiogenesis
* The process by which rounded nonmotile spermatids are converted to active and mobile sperms is called spermiogenesis.
1. The nucleus shrinks, chromosome become closely packed.
2. All nuclear material (except DNA) get lost.
3. The sperm nucleus becomes elongated and narrow.
4. The spermatid contain two centriole in the centrosome which move to lie behind the nucleus.
5. All mitochondria spirally arranged in the middle piece of spermatozoa.
6. The Golgi apparatus form acrosome.
7. Most of cytoplasm are eliminated and remaining forms a condensed layer surrounding the middle piece.
Sperm
* Each day about 300 million sperm complete the process of spermatogenesis.
* A sperm is about 60 μm long and contain several structures that are highly adapted for reaching and penetrating a secondary oocyte.
* The major parts of sperm are the head and the tail.
* The flattened. pointed head of the sperm is about 4 - 5 μm long.
* It contains a nucleus with 23 highly condensed chromosomes.
* Covering the anterior two-thirds of the nucleus is the acrosome, a caplike vesicle filled with enzymes that help a sperm to penetrate a secondary oocyte to bring about fertilization.
* Among the enzymes are hyaluronidase and proteases.
* The tail of sperm is subdivided into four parts : neck, middle piece, principal piece and end piece.
* The neck is the constricted region just behind the head that contains centrioles.
* The centrioles from the microtubules that comprise the remainder of the tail.
* The middle piece contains mitochondria arranged in a spiral, which provide the energy (ATP) for locomotion of sperm to the site of fertilization and for sperm metabolism.
* The principal piece is the longest portion of the tail and the end piece is the terminal, tapering portion of the tail.
* Once ejaculated, most sperm do not survive more than 48 hours within the female reproductive tract.
Oogenesis
* The formation of gametes in the ovaries is termed oogenesis.
* In contrast to spermatogenesis, which begins in males at puberty, oogenesis begins in females before they are even born.
* Oogenesis occurs in essentially the same manner as spermatogenesis, meiosis takes place and the resulting germ cells undergo maturation.
* During early fetal development, primordial (primitive) germ cells migrare from the yolk sac to the ovaries.
* There, germ cells differentiate within the ovaries into oogonia.
* Oogonia are diploid (2n) stem cells that divide mitotically to produce millions of germ cells.
* Even before birth, most of these germ cells degenerate in a process known as atresia.
* A few, however develop into larger cells called primary oocytes that enter prophase of meiosis I during fetal development due to not complete that phase until after puberty.
* Once a secondary oocyte is formed, it begins meiosis II but then stops in metaphase.
* The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process known as ovulation.
* At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the first polar body and corona radiata.
* Normally these cells are swept into the uterine tube.
* If fertilization does not occur, the cells degenerate.
* The secondary oocyte splits into two haploid cells, again of unequal size. The larger cell is the ovum, or mature.
* Egg : the smaller one is the second polar body.
* The nuclei of the sperm cell and the ovum then unite, forming a diploid zygote.
* If the first polar body undergoes another division to produce two polar bodies, then the primary oocyte ultimately gives rise to three haploid polar bodies, which all degenerate, and a single haploid ovum.
* Thus, one primary oocyte gives rise to a single gamete (an ovum).
* By contrast, recall that in males one primary spermatocyte produces four gametes (sperm).
Follicular development
* During this arrested stage of development, each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a primordial follicle.
* The ovarian cortex surrounding the primordial follicles consists of collagen fibers and fibroblastlike stromal cells.
* At birth, approximately 200,000 to 2,000,000 primary oocytes remain in each ovary.
* Of these, about 40,000 are still present at puberty and around 400 will mature and ovulate during a woman's reproductive lifetime.
* The remainder of the primary oocytes undergo atresia.
* Each month after puberty until menopause, gonadotropins (FSH and LH) secreted by the anterior pituitary further stimulate the development of several primordial follicles, although only one will typically reach the maturity needed for ovulation.
* A few primordial follicles start to grow, developing into primary follicles.
* Each primary follicle consists of a primary oocyte that is surrounded in a later stage of development by several layers of cuboidal and low-columnar cells called granulosa cells.
* The outermost granulosa cells rest on a basement membrane.
* As the primary follicle grows, it forms a clear glycoprotein layer called the zona pellucida between the primary oocyte and the granulosa cells.
* In addition, stromal cells surrounding the basement membrane begin to form an organized layer called the theca folliculi.
* With continuing maturation, a primary follicle develops into a secondary follicle.
* In a secondary follicle, the theca differentiates into two layers:
* The theca internal, a highly vascularized internal layer of cuboidal secretory cells that secrete estrogens, the theca externa, an outer layer of stromal cells and collagen fibers.
* In addition, the granulosa cells begin to secrete follicular fluid, which builds up in a cavity called the antrum in the center of the secondary follicle.
* The innermost layer of granulosa cells becomes firmly attached to the zona pellucida and is now called the corona radiata (corona crown; radiata radiation).
* The secondary follicle eventually becomes larger, turning into a mature (graafian) follicle.
* While in this follicle, and just before ovulation, the diploid primary oocyte completes meiosis I, producing two haploid (n) cells of unequal size - each with 23 chromosomes.
* The smaller cell produced by meiosis I, called the first polar body, is essentially a packet of discarded nuclear material.
* The larger cell, known as the secondary oocyte, receives most of the cytoplasm.
Structure of mature egg :
Functions of Egg :
* It supplies haploid set of chromosome to the future embryo.
* It supplies almost all the cytoplasm of the egg to the embryo.
* It supplies food to the developing embryo.
General Morphology :
* Compared to the sperm eggs are larger.
* Their size varies from being microscope to a few inches in diameter.
* In vertebrates, they range from 0.06 mm in diameter (Mouse) to 85 mm in diameter.
* The size of egg depends upon the amount of nutritive substance stored in the egg.
* Eggs are spherical in shape.
* The eggs are wall protected by egg membranes.
* The membranes are produced either by the egg itself or by the follicle cells of the ovary or by the genital duct.
* Accordingly egg membranes are classified into three types :
a. Primary membrane
b. Secondary membrane
c. Tertiary membrane
a. Primary membrane
* The membrane is secreted by the egg cytoplasm.
* They are closely attached to the surface of the egg.
1. Plasma membrane
2. Vitelline membrane
3. Chorion
4. Zona radiata
5. Zona pellucida
6. Jelly coat
1. Plasma membrane :
* It is the membrane covering the egg immediately over it.
* It is found in all the eggs.
* In structure, it resembles the plasma membrane of a typical cell.
2. Vitelline membrane
* It is closely attached to the plasma membrane of the egg.
* It is found in the egg of the majority animals like insects, molluscs, Echinodermata, amphi, etc.
* It is very transparent and thin.
* It is formed of mucopolysaccharide and fibrous protein.
* Immediately after fertilization, this membrane becomes separated and lifted of from the surface of the egg.
* After fertilization, this membrane is also called fertilization membrane.
* The space formed between P.M and it is called perivitelline space filled with a fluid perivitelline fluid.
Zona radiata :
* The egg of shark, has two primary membrane produced by the surface ooplasm.
* The outer membrane is the vitelline membrane and the inner membrane has radiating appearance and hence called zona radita.
* The eggs of the teleost fish is also covered by zona radiata.
Zona Pellucida :
* All mammalian eggs are surrounded by a membrane called zona pellucida.
* Is also named as zona radiata.
* Because it striated appearance.
* Striatation are due to the presence of microvilli and macrovilli.
* The microvilli are produced by the surface of the egg and macrovilli are produced by the follicle cells.
b. Secondary membrane :
* The S.M are produced by the follicle cells.
* These membrane usually tough and impermeable.
Corona radiata :
* It is found in mammalian eggs.
* This membrane is formed of a layer of follicle cells.
* The cells are radially arranged around the zona pellucida.
c. Tertiary membrane :
* Are produced by the oviduct.
* E.g. White albumen, shell membranes, shell and jelly coat.
Types of eggs :
a) On the basis of amount of yolk
b) On the basis of distribution of yolk
c) Presence of absence of shell
d) Types of development
a) On the basic of amount of yolk :
Eggs are grouped into three types on the basis of the amount of yolk present in them.
Alecithal :
* When the egg contains no yolk, it is called alecital egg.
* E.g. the eggs of eutherian mammals.
Microlecithal egg :
* When the egg contain small or negligible amount of yolk, it is said to be microlecithal.
* Romer and balinsky named these eggs as oligolecithal eggs.
* E.g. Amphioxus, tunicates.
Macrolecithal or Megalecithal egg :
* When the egg contain large amount of yolk, it is said to be macrolecithal or megalecithal egg.
* It is also called as polylecithal egg.
* e.g. bony fishes, amphibians, reptiles, birds, etc.
On the basis of distribution of yolk
* Eggs are classified into three types on the basis of the distribution of yolk in the egg.
1. Homolecithal or isolecithal :
* In isolecithal egg, the yolk material are uniformly distributed throughout the eggs.
* E.g. Echinodermata and amphioxus.
2. Telolecithal
* Here the yolk is highly concentrated towards the vegetal pole.
* The amount of the yolk gradually decrease from the vegetal pole towards the animal pole.
* e.g. fishes, amphibians, reptiles and birds.
3. Centrolecithal :
* In centrolecithal egg, the yolk takes up a central position and is surrounded by a thin layer of cytoplasm.
* E.g. Insect.