pregnancy and embryonic development
Embryonic development 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 plane). It divides the zygote completely 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 a transient 3-celled stage. Third cleavage is horizontal forming 8 blastomeres. It is slightly unequal. Thereafter the rate and pattern of cleavage is non specific. In mammals, including humans, cleavage divisions are among the slowest in animal kigdom. Also, the cleavage divisions are asynchronous. The number of resultant blastomeres increases following arithmetic progression.
- All cleavage divisions are mitotic and resultant daughter cells are blastomeres.
- During cleavage, there is no growth in the resulting blastomeres and the total size and volume of the embryo remains the same because in cleavage divisions interphase is without growth phase.
- During cleavage, the size of blastomeres keeps on decreasing as there is no growth of blastomeres. Zona pellucid remains intact throughout the cleavage divisions.
- During cleavage, there is no increase in mass of cytoplasm of the developing embryo. However, there is marked increase in the DNA content.
- The rate and type of cleavage depends upon the amount and distribution of yolk.
Morula:-
Cleavage results in a solid ball of cells called Morula having 8-16 cells. Zona pellucid still forms the outer cover. Morula undergoes compaction. The outer/peripheral cells are smaller/flat with tight junctions while the inner cell mass consists of slightly large, rounded cells with gap junctions. Morula descends slowly towards uterus in 4-6 days and corona radiate detaches during this period.
Blastulation or blastocyst Formation:–
Endometrium secretes a nutrient rich fluid and its mucosal cells become enlarged with stored nutrients. As the morula enters uterus, it gets a rich supply of nutrients. Outer peripheral cells enlarge and flatten further. They form trophoblast or trophoectoderm. Trophoblast cells secretes a fluid into the interior creating a cavity called blastocoels. 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.
Due to pressure of growing blastocyst, a slit is produced in zona pellucid through which it squeezes out. The growing blastocyst comes out of this slit. At times, it gets broken into two parts which then gives rise to identical twins or monozygotic twins.
Trophoblast cells in contact with embryonal knob are called cells of Rauber. Area of embryonal knob represents animal pole. The opposite side is abembryonal pole. Soon embryonal knob shows rearrangement to form embryonal disc. This gives rise to two layers, outer syncytiotrophoblast and inner cytotrophoblast. The two layers form chorion, amnion and foetal part of placenta.
Implantation:-
It is embedding of the balstocyst into endometrium of uterus. Blastocyst comes in contact with the endometrium in theregion of embryonal knob or embryonic disc and adheres to it. The surface cells of trophoblast secretes lytic enzymes which cause corrosion of endometrium lining. They also gives rise to finger-like outgrowths called villi. Villi not only help in fixation but also in absorption of nutrients. Implantation causes nutrient enrichment, enlargement of cells and formation of uterine part of placenta called deciduas (L.deciduos-falling off).
Decidua has three regions:-
(i) Decidua Basalis:- part of deciduas underlying the chorionic villi and overlying the myometrrium.
(ii) Decidua capsularies:– Lying between embryo and lumen of uterus.
(iii) Decidua parietalis:- The part of deciduas that lines the uterus at places other than the site of attachment of embryo.
Trophoblast covering secretes a 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 time when it is called corpus leteum of pregnancy. It continues to secrete progesterone which prevents menstruation and maintains the uterine lining in nutrient rich state. Progesterone induces the cervical glands to secrete viscus 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 secrete by placenta as well.
Gastrulattion:-
It is characterised by movements 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.
A space appears between the ectoderm (below) and the trophoblast. This is the amniotic cavity, filled by amniotic fluid. The roof of this cavity is formed by amniogenic cells derived from trophoblast.
Formation of Primary Germinal Layers:- Cells of the inner cell mass or embryonal knob get rearrangement to form a flat embryonic or germinal disc. The latter differentiates into layers, an outer epiblast of larger columnar cells and inner hypoblast of smaller cuboidal cells.
- Gastrulation begins with the formation of primitive streak on the surface of the epiblast.
- Cranial region of the streak at 15 days shows movement of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm.
- Once definitive endoderm is established, inwardly moving epiblast forms mesoderm.
- Cells remaining in the epiblast then form ectoderm, Thus the epiblast is the source of all the germ layers in the embryo.
A blastocyst is a ball of cells with large, fluid-filled cavity called blastocoels. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and inner mass of cells (attached to trophoblast) called the inner cell mass.
Placenta:-
After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the 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. Human placenta is described as haemo-chorial, deciduous and meta –discoidal.
Placentais an organ which connects the foetus with uterine wall. It constitutes both maternal as well as foetal parts although which soluble inorganic and organic materials, nutrients, hormones, antibodies against diphtheria, small pox, scarlet fever and measles etc. can pass from the mother to the foetus.
Placenta also acts as an endocrine gland and synthesizes large quantities of proteins and some hormones such as human chorionic gonadotropin (hCG), chorionic thyrotropin, chorionic corticotrophin, chorionic somatomammotropin or human placental lactogen (hPL), estrogen and progesterone. The hCG-stimulates corpus luteum during
pregnancy to continue to secrete progesterone for a long time after its normal life time i.e until placenta is well established. In addition, corpus leteum secretes some relaxin that facilitate parturition by softening the connective tissue of the of the pubic symphsis. The meta bolic activity of the placenta is almost as great as that of the foetus itself. The umbilical cord connects the foetus to the placenta. This involves cell division, cell migration and the differentiation of cells into the many types found in the body. During this period, the developing baby called foetus is very sensitive to anything that interferes with the steps involved.
Virus infection in mother e.g by Rubella (German measles) virus or exposure to certain chemicals may cause malformations in the developing embryo. Such agents inducing malformations are called teratogens (monster forming agents). By 3 months, all the systems of the foetus have been formed, at least in a rudimentary way. From then on, development of the foetus is primarily a matter of growth and major structure modifications. The foetus is less susceptible to teratogens after first trimester.
Organogenesis:-
The human pregnancy lasts 9 months. The gestation period of a dog is 60-65 days, for an elephant it is 607-641 days and in cat it lasts 52-65 days. In human beings, after one month of pregnancy, the embryo’s heart is formed. The first sign of growing foetuus may be noticed by listening to the heart sound carefully through the stethoscope. By the end of the second month of pregnancy, the foetus develops limbs and digits. By end of 12 weeks (first trimester), most of major organ systems are formed, for example, the limbs and external genital organs are well-developed. The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month. By the end of 24 weeks (second trimester), the body is covered with fine hair, eye-lids separate and eyelashes are formed. By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery.
Yes, moderate exercise is generally safe and beneficial during pregnancy. Activities such as walking, swimming, and prenatal yoga are recommended. Always consult with your healthcare provider before starting any exercise routine.
Complications can include gestational diabetes, preeclampsia, preterm labor, and miscarriage. Regular prenatal care is crucial for monitoring and managing these risks.
A type of diabetes that develops during pregnancy. It can often be managed with diet and exercise but sometimes requires medication.
Monthly visits until 28 weeks, bi-weekly from 28 to 36 weeks, and weekly from 36 weeks to delivery.
Prenatal vitamins typically contain folic acid, iron, calcium, and DHA. Folic acid is crucial to prevent neural tube defects.
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