EMBRYONIC DEVELOPMENT

It includes cleavage, blastulation, implantation, gastrulation and organogenesis.

Cleavage:

First cleavage is completed after 30 hours of fertilization. Cleavage furrow passes from animal-vegetal axis as well as centre of zygote (Meridional cleavage).

It divides the zygote into two blastomeres (Holoblastic cleavage). Second cleavage is completed after 60 hours of fertilization.

It is also meridional but at right angle to the first one. It is completed earlier in one of the two blastomeres resulting in transient 3-celled stage.

Third cleavage is horizontal forming 8 blastomeres. lt is slightly unequal. Thereafterthe rate and pattern of cleavage is not specific.

Morula:

Cleavage results in solid ball of celled Morula with 16 cells (occasionally 32 cells). Zona pellucida is still present as the outer cover. Morula undergoes compaction.

The outer/peripheral cells are small/flat with tight junction while the inner cell mass slightly large round and with gap junction.

Morula descends slowly towards uterus in 4-6 days and corona radiata detaches during this period.

BLASTULATION OR BLASTOCYST FORMATION

Endometrium secretes a nutrient fluid and its mucosal cells become enlarged with stored nutrients. As the morula enters uterus, it obtains enriched supply of nutrients.

Outer peripheral cells enlarge and flatten further. They form trophoblast or trophoectoderm. Trophoblast secretes a fluid into the interior. It creates a cavity called blastocoel.

The inner cell mass now comes to lie on one side as embryonal knob.

With the formation of blastocoel, morula is converted into blastula which is called blastocyst in mammals because of different nature of surface layer and eccentric inner cell mass.

Development of blastocyst

Due to pressure of growing blastocyst a slit is produced in zona pellucida. The growing blastocyst comes out. At times it gets broken into two parts which then give rise to identical twins.

Trophoblast cells in contact with embryonal knob are called cells of Rauber. Area of embryonal knob represents animal pole.

The opposite side is embryonal pole. Soon embryonal knob shows rearrangement to form embryonal disc. Cells of trophoblast layer divide periclinally.

This makes trophoblast two layered, outer syncytotrophoblast and inner cytotrophoblast. The two layers later form chorion, amnion and foetal part of placenta.

IMPLANTATION

It is embedding of the blastocyst into endometrium of uterus.

Blastocyst comes in contact with the endometrium in the region of embryonal knob or embryonic disc. It adheres to the same.

The surface cells of trophoblast secrete lytic enzymes which cause corrosion of endometrial lining.

They also give rise to finger-like outgrowths called villi. Villi not only help in fixation but also absorption of nourishment.

Implantation causes nutrient enrichment, enlargement of cells and formation of uterine part of placenta called decidua (L. deciduus-falling off).

Decidua has three regions (i) Decidua Basalis (Basal Decidua, Tunica Serotina). Part of decidua underlying the chorionic villi and overlying the myometrium. (ii) Decidua Capsularis (Decidua Reflexa). It lies between embryo and lumen of uterus. (iii) Decidua Parietalis (Decidua Vera).

It is part of decidua that lines the uterus at a place other than the site of attachment of embryo.

Trophoblast secretes hormone called human chorionic gonadotropin (hCG). Detection of hCG in the urine is the basis of pregnancy/Gravidex test.

hCG maintains the corpus luteum beyond its normal life. It continues to secrete progesterone which prevents menstruation and maintains the uterine lining in nutrient rich state.

Progesterone induces the cervical glands to secrete viscous mucus for filling the cervical canal to form a protective plug.

Progesterone is also called pregnancy hormone as it is essential for maintenance of pregnancy. The hormone is secreted by placenta as well.

GASTRULATION

It is characterised by movement of cells in small masses or sheets so as to form primary germinal layers. There are three primary germinal layers -endoderm, ectoderm and mesoderm.

The cell movements that occur during gastrulation are called morphogenetic movements since they lead to initiation of morphogenesis. The product of gastrulation is called gastrula.

 Formation of endoderm, and amniotic cavity

FORMATION OF PRIMARY GERMINAL LAYERS

Cells of the inner cell mass or embryonal knob get rearranged to form a flat embryonic or germinal disc. The latter differentiate into two layers, outer epiblast of larger columnar cells and inner hypoblast of smaller cuboidal cells.

Gastrulation begins with the formation of the primitive streak on the surface of the epiblast.

1. Cross section through the cranial, region of the streak at 15 days showing movement of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm.

2. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm.

3. Cells remaining in the epiblast form ectoderm. Thus the epiblast is the source of all the germ layers in the embryo.

FATE OF GERM LAYERS

PLACENTA

Placenta is an organ which connects the foetus and uterine wall.

It is contributed by both-maternal as well as foetal part although there is no blending of the maternal and foetal blood supplies, the placenta acts as an ultrafilter, soluble inorganic and organic materials nutrients, hormones, antibodies against diphtheria, small pox, scarlet fever, measles, etc. can pass from the mother to the foetus.

Placenta acts as an endocrine gland and synthesises large quantities of proteins and some hormones, such as human chorionic gonadotropin (hCG), chorionic thyrotropin, chorionic corticotropin, chorionic somatomammotropin, estrogens and progesterone.

The hCG stimulates the corpus luteum to secrete progesterone until the end of pregnancy. In addition, it secretes relaxin that facilitates parturition by softening the connective tissue of the symphysis pubica.

The metabolic activity of the placenta is almost as great as that of the foetus itself. The umbilical cord connects the foetus to the placenta.

During the first trimester (first 3 months) of pregnancy, the basic structure of the baby is formed.

This involves cell division, cell migration, and the differentiation of cells into many types found in the baby. During this period, the developing baby-called foetus is very sensitive to anything that interferes with the steps involved.

Virus infection of the mother e.g., by Rubella (German measles) virus or exposure to certain chemicals, may cause malformations in the developing embryo. Such agents are called teratogens (monster forming).

By 3 months, all the systems of the baby have been formed, at least in a rudimentary form.

From then, development of the foetus is primarily a matter of growth and minor structural modifications.

The foetus is less susceptible to teratogens.

Table-I: Important Developmental Events in the Human Embryo

* Lanugo is the soft hairy covering of the foetus which begins to be shed before birth.