PREGNANCY AND EMBRYONIC DEVELOPMENT

The fusion of sperm and ovum (egg) is called fertilisation and the attachment of zygote (fertilised egg) in the endometrium of uterus (womb) is called implantation which leads to pregnancy. Zygote is the fertilised egg. Pregnancy is the period from the fertilization of egg up to parturition (Child birth).  Embyonic development starts just after fertilisation. It includes cleavage, blastulation, implantation, gastrulation and organogenesis. First cleavage completed after 30 hours of fertilisation. It divides the zygote into two blastomeres (Holoblastic cleavage). Second cleavage is completed after 60 hours of fertilisation. It is completed earlier in one of the blastomeres resulting transient 3-celled stage. Third cleavage results in the formation of 8 blastomeres. It is slightly unequal. Thereafter the rate and pattern of cleavage is non-specific. Cleavage results in a solid ball of cells called MORULA having 16 cells (occasionally 32 cells). Zona pellucida forms outer covering. Morula undergoes compaction, Morula slowly descends towards uterus in 4-6 days and corona radiates detaches during this period. 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 EMRYONAL KNOB. With the formation of blastocoel, morula converted into BLASTULA. Trophoblast cells in contact with embryonal knob are called CELLS OF RAUBER.

After implantation, finger like projections appear on the trophoblast called chorionic villi which are surrounded by uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (Foetus) and maternal body called PLACENTA. The placenta facilitates the supply of oxygen and nutrients to the embryo and removal of carbondioxide and excretory/waste materials produced by the embryo. The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo. It also acts as an endocrine tissue and produces several hormones like HUMAN CHORIONIC GONADOTROPHIN (HCG), HUMAN PLACENTAL LACTOGEN (HPL), ESTROGEN, PROGESTOGENS etc. In the later phase of pregnancy, a hormone called RELAXIN is also released from the ovary. HCG, HPL and RELAXIN are produced in women only during pregnancy. The level of other hormones like estrogens, progestrogens, cortisol, prolactin, thyroxine etc are also increase several times in maternal blood during pregnancy. Increased production of these hormones is essential for supporting the foetal growth, metabolic changes in mother and maintenance of pregnancy. Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ECTODERM and an inner layer called ENDODERM. A middle layer called MESODERM soon appears between the ectoderm and endoderm. These three layers layers give rise to all tissues (organs) in adults. It needs to be mentioned that the inner cell mass contain certain cells called stem cells which have the potency to give rise to all tissues and organs.

The human pregnancy lasts for nine months. The embryo’s heart is formed after one month of pregnancy. The foetus develops limbs and digits at the end of two months of pregnancy. Most of the organ systems are formed after the 12 weeks of pregnancy. The first movement foetus and appearance of hairs on head observed during the fifth month of pregnancy. By the end of 24 weeks (second trimester), the body is covered with fine hairs, eye lids separate and eyelashes are formed. By the end of nine month, the foetus is fully developed to be delivered.

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