Question

In mammals the estrogens are secreted by the Graafian follicle from its
A) External theca
B) Internal theca
C) Zona pellucida
D) Corona radiata

Answer 1

Hint: Estrogen is the female sex hormone which is responsible for the growth and development of secondary sexual characters like breast, pubic hairs. It also helps in keeping the bones strong. The low estrogen leads to decreased bone density.

Complete answer:
The graafian follicle is a fluid filled cavity which is present adjacent to the oocyte. It contains Granulosa and theca cells which continuously divide. It produces estrogen.

The production of estrogen requires both the cells of the theca interna and granulosa cells. Estrogens, like all steroid hormones, are produced from cholesterol through a multi-step process that needs several different enzymes. Neither the cells of the theca interna nor the granulosa cells contain all of the enzymes necessary to convert cholesterol into estrogens.

Theca cells contain enzymes that catalyze the initial conversion of cholesterol into androgens but lack aromatase that carries out the ultimate steps of converting androgens into estrogens. Consequently, androgens produced by theca cells diffuse into the granulosa cells which contain aromatase but lack the enzymes for the initial steps in estrogen synthesis. The theca cells are in a very better position to catalyze the initial steps in estrogen synthesis because they're closer to blood vessels and might take up LDL to get cholesterol.

i) Theca externa is a layer of connective tissue which is derived from fibroblasts which contains extracellular matrix factors like collagen.
ii) Zona Pellucida- It is the glycoprotein layer surrounding the mammalian oocyte. It regulates interaction between eggs and sperm.
iii) Corona radiata- It is the innermost layer of cells adjacent to zona pellucida.

Therefore, the correct answer is option ‘B’, internal theca.

Note: The theca interna is an endocrine gland. The theca interna contains luteinizing receptors. It secretes estrogen in response to the luteinizing hormone. The LH is regulated by the pituitary.