12th Class Biology Human Reproduction Implantation

(i) Definition: The process of attachment of the blastocyst on the endometrium of the uterus is called implantation.

(ii) Period: Though the implantation may occur at any period between 6^th and 10^th day after the fertilization but generally it occurs on seventh day after fertilization.

(iii) Mechanism: First of all, the blastocyst is held closely against the uterine endometrial epithelium. The uterine capillaries and uterine wall in the immediate vicinity of the embryo become more permeable and a local stromal edema is developed. Soon the endometrium around the embryo shows the first sign of a decidual cell reaction (DCR) which involves:

(a) The epithelium becomes disrupted and the loosely packed fibroblast-like cells of the stoma are transformed into large rounded glycogen-filled cells.

(b) The area of contact becomes more vascular.

(c) The decidual cells form an “implantation chamber” around the embryo before the formation of a functional placenta.

(d) The tropho blast is developed from the superficial layer of the morula stage. Later, the trophoblast is lined by mesoderm to form the chorion which contributes to the placenta formation.

(e) Trophoblast of the chorion penetrates the uterine epithelium by both cytolytic and mechanical activity. The phagocytic activity of the trophoblastic cells through the decidual cells continues till it establishes intimate connection with the uterine blood vessels. The process of implantation is aided by proteolytic enzymes produced by the trophoblast. After implantation, endometrium undergoes many changes and forms decidua. It is differentiates into three parts such as : Decidua basalis present between the embryo and uterine myometrium, Decidua capsularis lies between the embryo and lumen of the uterus and Decidua parietalis is formed by the remaining part of decidua. The pattern of implantation of the blastocyst varies in different species, which are as follows

(1) Interstitial implantation: The blastocyst get burried into the endometrium e.g. human female, hedgehog, guinea pig, some bats and ape.

(2) Central implantation: The blastocyst remain the uterine cavity e.g. rabbit, cow, dog and monkey.

(3) Eccentric implantation: The blastocyst comes to lie in a uterine recess e.g. rats, mice.

(iv) Hormonal control of implantation

(a) Role of estrogens: These are a group of steroid hormones mainly secreted by follicular epithelial cells of Graafian follicle though these are also produced by adrenal cortex and placenta. These include b-estradiol, esterone, estriol etc. Out of which most important estrogen is b-estradiol. Secretion of estrogens is stimulated by FSH of anterior lobe of pituitary glands. These stimulate the uterine endometrial epithelium to enlarge, become more vascular and more glandular. The uterine glands become tortuous and cork-screw shaped. So the endometrium prepares itself for implantation. This stimulation by the estrogens on the uterus generally occurs on the 4th day of pregnancy.

(b) Progesterone: It is also a steriod hormone secreted by yellow-coloured endocrine gland, called corpus luteum, formed from empty Graafian folicle during the pregnancy. Small amount of progesterone is also secreted by adrenal cortex and placenta. Secretion of progesterone is stimulated by LH of anterior lobe of pituitary gland.

Progesterone acts on only those uterine cells which have been earlier stimulated by estrogens. Progesterone further stimulates the proliferation of endometrium of uterus and prepared, placenta formation and normal development of the foetus in the uterus.