HPG Axis and Early Follicle Growth

Vocabulary flashcards covering hormones, axis, oogenesis, and follicle development topics from the lecture notes.

Hormone

A chemical signal produced by an endocrine gland, transported in the blood, that acts on distant target organs at very low concentrations.

Protein hormone

Hydrophilic hormones synthesized in the ER, stored in secretory vesicles, released into capillaries, and binding to transmembrane receptors.

Steroid hormone

Lipid-soluble hormones made from cholesterol in mitochondria and cytosol; diffuse into cells and bind intracellular receptors; often travel with carrier proteins in blood.

Lipid hormone

Hormones made from fatty acids, synthesized in the cytosol; lipophilic; require transport proteins and bind to transmembrane receptors.

Estrogen

Primary female sex hormones; estradiol is the main estrogen produced by the ovaries; placenta makes estriol and estrone; phytoestrogens and synthetic estrogens exist.

Progestin (Progestogen)

Class of pro-gestational hormones; main is progesterone; synthetic progestogens (e.g., MGA, norethindrone); maintain pregnancy and help prevent ovulation.

GnRH

Gonadotropin releasing hormone from the hypothalamus; stimulates the anterior pituitary to secrete FSH and LH.

FSH

Follicle-stimulating hormone from the pituitary; promotes growth of ovarian follicles and estrogen production.

LH

Luteinizing hormone from the pituitary; triggers ovulation and stimulates theca cells to produce androgens.

Estradiol

Main estrogen produced by ovarian follicles; promotes ovarian/uterine changes, triggers LH surge, and provides feedback to the hypothalamus.

Inhibin

Hormone produced by growing follicles that inhibits FSH secretion, contributing to negative feedback.

Activin

Hormone that stimulates FSH production and participates in the feedback regulation of the pituitary.

HPO axis

Hypothalamic‑Pituitary‑Ovarian axis: GnRH from hypothalamus stimulates FSH/LH from the pituitary; FSH/LH stimulate ovarian estradiol and progesterone; ovarian steroids modulate GnRH.

Primordial germ cells (PGCs)

Diploid precursors of oocytes and sperm; originate from yolk sac and migrate to the genital ridge.

Meiosis (female)

Process by which oocytes reduce chromosome number; PGCs enter meiosis, arrest at certain stages, and yield one mature oocyte with polar bodies after completion.

Primordial follicle

Early ovarian follicle formed when oocytes recruit surrounding cells to become pre-granulosa cells; millions are present at birth and arrest in development.

Antral follicle

Follicle that has developed an antrum (fluid-filled cavity); a later stage before ovulation in the follicle growth sequence.

Graafian follicle

Mature, dominant follicle selected for ovulation; produces high estradiol and suppresses others.

Atresia

Degeneration and death of follicles; most follicles die during development, reducing the ovarian pool.

2-Cell 2-Gonadotropin Theory

Theca cells produce testosterone in response to LH; granulosa cells convert testosterone to estradiol under FSH; both cell types are needed for estradiol production.

Cohort of follicles

Group of developing follicles recruited to grow together, producing estradiol and inhibin; most die by atresia, one becomes dominant.

Dominant follicle

Follicle that continues to grow in response to rising estradiol and suppresses other follicles, leading to ovulation.

Gonadotropin-dependent growth

Follicle growth that requires gonadotropins (FSH/LH) to continue, especially after puberty.

Gonadotropin-independent growth

Initial follicle growth that occurs without gonadotropin support; occurs before cohort becomes gonadotropin-dependent.

Oocyte

Mature egg cell produced through meiosis; one oocyte results from most PGCs, accompanied by polar bodies during meiosis.