Embryology is the study of developmental changes (structurally and physiologically) that a zygote goes through till formation of the adult form. Study of all the sequential systematic procedures includes developmental biology or embryology. Developmental biology comprises the study of (i) ontogenic development that involves transformation of fertilized egg into new adult. (ii) phylogenetic development that involves steady transformation of forms of life. Embryonic development comprises fertilization, gametogenesis, blastulation, cleavage, gastrulation and organogenesis.
It comprises fusion of gametes to create diploid zygote. Yolk is the morphological term utilized to explain reserve food in oocyte, neutral fats, formed of proteins, phospholipids, carbohydrates and glycogen. Procedures of synthesis and accumulation of yolk is known as as vitellogenesis. Based on distribution and amount of yolk eggs are categorized in following categories.
I) Based on Amount of Yolk:
1. Microlecithal: Small sized egg having only a small amount of yolk or reserve food. Like Amphioxus, sea urchin, starfish. According to some scientists, alecithal term is also utilized, meaning no yolk, but this term is not suitable as no egg is without yolk.
2. Mesolecithal: Eggs having reasonable amount of yolk. Like amphibians.
3. Macrolecithal: Eggs having huge amount of yolk. Like reptiles, birds.
II) Based on Distribution of Yolk:
1. Isolecithal: In microlecithal eggs amount of yolk is very little that it is equally distributed throughout the egg cytoplasm. Like protochordates and echinoderms.
2. Telolecithal: In mesolecithal and macrolecithal eggs yolk is concentrated in lower part of egg (vegetal pole), whereas upper part is yolk free known as animal pole. Like reptiles and birds.
3. Centrolecithal: In macrolecithal eggs the yolk is concentrated in centre of the egg with cytoplasm surrounding it. Like insects.
Kinds of Egg Membranes:
I) Primary Egg Membranes: Formed outside plasma membrane of the oocyte secreted by the follicle cells. These are of given kinds:
1. Vitelline membrane: Closely applied to plasma membrane, found in insects, molluscs, amphibians and birds formed of mucopolysaccharides and fibrous proteins.
2. Zona radiata: It is striated in appearance or perforated by tiny pores, discovered in sharks, bony fishes, few amphibians and reptiles.
3. Zona pellucida: It is unstriated in appearance, created by secretions of ovum and the follicle cells, found in mammals.
4. Jelly envelope: It is jelly like and thicker in appearance also called as jelly coat, found in echinoderms and marine invertebrates.
II) Secondary Egg Membranes: Secreted outside primary egg membranes by follicle cells surrounding the oocyte. Like chorion present around the egg of insect. In mammalian eggs secondary egg membrane is absent; instead the layer of follicle cells surrounds zona pellucida called as corona radiata. It is not a true membrane as its cells are peeled off as egg passes through oviduct.
III) Tertiary Egg Membranes: Secreted either by oviduct or some other accessory parts of maternal genital tract when egg passes down by oviduct to the exterior. Like outermost calcareous shell of hen's egg, jelly coat around amphibian oocyte.
All mammals depend on internal fertilization by copulation. To deliver sperm to the female, male inserts his sexual organ, the penis, in opening of the vagina; the way in the female's other sexual organs. Once male ejaculates, a lot of sperm cells swim from upper vagina through cervix and across the length of uterus toward ovum- a significant distance compared to size of sperm cell. Spermatozoon and oocyte meet and interact in ampulla of fallopian tube. It is probable that chemotaxis is involved in directing sperm to the egg, but device has yet to be worked out. Human fertilization is union of the human egg and sperm, generally occurring in ampulla of fallopian tube. There is specific sequence of events which take place in fertilization:
i) The sperm passes through corona radiata, the outermost cell layer of egg.
ii) The sperm breaks through zona pellucida. This takes place with the help of many enzymes possessed by sperm which break down proteins of zona pellucida, the most significant one being acrosin.
iii) Cell membranes of egg and sperm fuse together.
iv) Female egg, also known as a secondary oocyte at this phase, completes its second meiotic division. This results in the mature ovum.
v) Sperm's tail and mitochondria degenerate with formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
vi) Male and female pronuclei fuse to form the new nucleus which is the combination of genetic material from both sperm and egg.
Mechanism of Fertilization:
(a) Approximation of Sperm and ovum: This is done by fertilizin-antifertilizin compatibility reaction. The chemical substance, fertilizin secreted from cortical region of egg cytoplasm interacts with the antifertilizin of sperm of the same species. This makes sperms attach to surface of the egg. Both fertilizin and antifertilizin are species particular; fertilizin serves as a receptor for antifertilizin and makes sperm capable of fertilizing egg of same species. This procedure is known as capacitation.
(b) Acrosomal Reaction: Acrosome is the extremely altered lysosome derived from golgi apparatus during spermiogenesis. Sperm, after attachment to egg surface has to penetrate egg membranes in order to reach egg plasma membrane. Acrosome has given enzymes that help in fertilization:
i) Zona lysins: These are proteolytic enzymes generated from acrosome and are capable of dissolving zona pellucida and clear path for sperm to reach the plasma membrane of the egg.
ii) Hyaluronidase: The mammalian egg is covered by the membrane known as corona radiata, formed of single layer of follicular cells, connected together by the adhesive substance, hyaluronic acid. Hyaluronidase dissolves hyaluronic acid, separates follicular cells of corona radiata so that sperm can push inward. Acrosome is not able to release hyaluronidase and zona lysins till it has gone through capacitation.
iii) Neuraminidase: It is the hydrolytic enzyme that checks entry of more than one sperm from entering ovum and therefore prevents polyspermy.
(c) Activation of Ovum: As the sperm enters ovum (really a secondary oocyte), it gets activated and goes through second meiotic division forming the haploid sperm and secondary polar body. This is followed by breakdown of nuclear membrane of sperm and ova at their point of contact and their contents gets enclosed by common nuclear membrane forming diploid zygote nucleus, that has maternal and paternal chromosomes. Zygote starts to divide and form blastocyst and when it reaches uterus, it implants in endometrium. At this point female is said to be pregnant. If embryo emplants in any tissue other than the uterine wall which results an ectopic pregnancy and it can be fatal to the mother.
Immediately after fertilization, fertilized egg goes through a series of repeated mitotic cell divisions to create multicellular blastula. Cleavage can be stated as procedure of progressive subdivision of zygote by mitotic cell divisions in the increasing number of cells of gradually decreasing size. Cleavage varies from mitosis in the given aspects:
There is no growth stage between successive divisions, so resulting blastomeres are half the original size. As, growth stage is absent, interphase is extremely short. Metabolic activity is very high comprising high consumption of oxygen and rapid DNA synthesis.
Cleavage continues till reserves are exhausted and average size of the daughter cells (blastomeres) reaches the characteristic size of the distinguished somatic cells of parent organisms. Pace of cleavage is found by cytoplasm rather than by nucleus. Nuclear-cytoplasmic ratio is very high at the starting of the cleavage; it slowly increases during cleavage and at the end it is brought to same level as is found in the ordinary somatic cells. Rate of cleavage is rapid in early development and is synchronous, but it becomes very slow and asynchronous by completion of blastula.
Kinds of Cleavage:
Yolk is the non-living component of egg that don't take part in cleavage or formation of embryo. Its function is only to give nourishment to developing embryo. Yolk has the pronounced influence on procedure of cleavage; retarding and interfering with it. Cleavage takes place more rapidly inactive yolk-free cytoplasm than in yolk-laden cytoplasm, as yolk granules remain inert and passive in cleavage. Thus, blastomeres that are richer in yolk remain larger in size than those containing less yolk. Depending on amount and distribution of yolk, cleavage may be holoblastic and meroblastic.
i) Holoblastic: In this kind of cleavage, cleavage pass through complete egg, dividing it totally in equal or about equal blastomeres. It takes place in alecithal, microlecithal and mesolecithal eggs. It is of two kinds: Holoblastic equal- It takes place in mesolecithal or telolecithal eggs, where the yolk is spread along vegetal animal axis and blastomeres are of unequal size like Amphioxus, marsupials and placental mammals. Holoblastic unequal- It takes place in microlecithal or isolecithal eggs and blastomeres are of equal size known as micromeres (small) and macromeres (large) like amphibians and lower fishes.
ii) Meroblastic: Such kind of cleavage takes place in megalecithal or heavily telolecithal eggs, that have huge amount of yolk. Active portion of egg is confined to the small cytoplasmic region at animal pole, known as germinal disc or blastodisc. Cleavage furrow don't entirely pass through egg and remains confined to germinal disc as yolk gives restriction to cleavage like insects, reptiles.
Planes of Cleavage:
i) Meridional: Cleavage furrow passes by polar axis of the egg and bisects both animal and vegetal pole through the middle like amphibians.
ii) Vertical: Cleavage furrow passes through animal and vegetal pole but not through median axis. It passes either by left or right of the axis like chick.
iii) Equatorial: Cleavage furrow is horizontal and is laid down in equatorial plane at right angles to main axis between animal and vegetal poles like mammals.
iv) Latitudinal: Cleavage furrow is laid down transversely or horizontally not on equator but on either side of it like Amphioxus.
Patterns of Cleavage:
The formation of the segmentation cavity or blastocoel inside the mass of cleaving blastomeres and rearrangement of blastomeres around the cavity in such a way as to create the kind of definitive blastula is the characteristic of each species. Blastocoel originates as the intercellular space that at times arises as early as four or eight-cell stage. Therefore blastulation is started in early cleavage stages, and formation of definitive blastula is thought to end cleavage and to start gastrulation. Blastula is the untimely phase of embryonic development in animals. It is also known as blastosphere.
In mammals, blastulation leads to the formation of the blastocyst that should not be confused with blastula; although they are similar in structure, their cells have different fates. Blastocyst comprises of three parts: inner cell mass, trophoblast and blastocoel. Inner cell mass is inner cluster of blastomeres, that is source of embryonic stem cells and gives rise to all later structures of adult organism. Trophoblast combines with maternal endometrium to create the placenta in eutherian mammals.
Before gastrulation, embryo implants in uterus. Blastocyst hatches from zona pellucida at about the time it enters uterus. Trophoblast cells join to endometrial cells of the uterus by adhering to extracellular matrix molecules like collagen, fibronectin, hyaluronic acid, and heparan sulphate receptors. After contact, cells directly in contact with endometrium fuse to create syncytiotrophoblast whereas remaining trophoblast cells are known as cytotrophoblast cells. Syncytiotrophoblast cells generate enzymes which permit for invasion of embryo in uterine wall and to induce essential changes in uterine tissue. Inner cell mass, in the meantime, divides in two layers: epiblast and hypoblast. Hypoblast spreads out and covers blastocoel to create yolk sac. Yolk sac is the extraembryonic tissue which produces blood cells like the structure which surrounds yolk in birds. Epiblast further divides in two more layers. Amnion layer forms fluid filled cavity to surround and protect embryo during pregnancy. Embryonic epiblast goes through gastrulation.
During gastrulation a cavity known as archenterone happens inside gastrula forming future alimentary canal. All morphogenetic movements in gastrulation are divided in following categories:
(a) Epiboly: It is the spreading of ectodermal cells (micromeres) over secretly moving endodermal and mesodermal cells (macromeres). Micromeres divide rapidly to create large number of cells that migrate downwards and distribute over the macromeres.
(b) Emboly: It involves inward movement of mesodermal and endodermal cells, that are different in different animals, as follows-
(c) Invagination: It is inward folding of endodermal cells to create archenterone.
(d) Involution: It is inward rotation of mesodermal and endodermal cells from surface of blastula to go in blastocoel.
(e) Delamination: Cells split off from pre-existing layer of mass in blastocoel. These separated cells create hypoblast and the blastopore is not formed.
Gastrulation results in establishment of three main germ layers that is, ectoderm, mesoderm and endoderm. All organs of embryo are formed from these layers. Development of morphologically identifiable tissues and organs of new individual is known as organogenesis.
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