PREGNANCY AND EMBRYONIC DEVELOPMENT

After implantation, finger like projections appear on the trophoblast called as chorionic villi.
Uterine tissue and maternal blood surrounds the chorionic villi.
The chorionic villi and uterine tissue together form a structural and functional organic structure between developing embryo and tissues of the mother called as placenta

fig. foetus with placenta

Functions of placenta-

The placenta facilitates the supply of oxygen and nutrients to the embryo.
Help in the removal of carbon dioxide 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.
Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG),human placental lactogen (hPL),estrogens, progestogen. etc.
A hormone called relaxin is secreted by the ovary in the later phase of pregnancy.
hCG, hPL and relaxin are produced in women only during pregnancy.
Levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood.
Increased production of all the hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.
After implantation, the inner cell mass is differentiated into outer layer called ectoderm and an inner layer called endoderm with a middle
Three layers give rise to all organs in adults.
The cells which have the potency to give rise to any types of cells in the body are called stem cells.
The human pregnancy lasts for 9 months, heart develops after one month of pregnancy, limbs develop by the end of second month,major organ systems are formed by the end of 3 months.
First movement and appearance of hairs are during fifth month of pregnancy.
By the end of 24 weeks, the body covers with fine hair, eye-lids separate, eyelashes form.
By the end on nine month, the fetus fully develops.