Unit 17: Human Reproductive Systems — Short Questions

The two seminal vesicles release a fructose-rich fluid into the vas deferens; this sugar provides energy for the sperm. The prostate gland lies below the urinary bladder and produces about 60% of the fluid volume of semen (semen being a mixture of sperm with fluids from the seminal vesicles and prostate). The two bulbourethral glands secrete a clear fluid that lubricates the urethra for the passage of sperm and provides a protective, neutral environment in which sperm can survive.

The primary oocytes begin the first meiotic division (meiosis-I) during embryonic development of the female, but the division stops at prophase-I, and they remain arrested there throughout childhood for at least 10 to 14 years. Only at puberty does one primary oocyte resume and complete meiosis-I each cycle, producing a secondary oocyte and the first polar body.

FSH (follicle-stimulating hormone), released by the anterior pituitary, stimulates the seminiferous tubules to carry out spermatogenesis; it also causes Sertoli cells to release inhibin, which inhibits further FSH secretion. LH (luteinizing hormone) stimulates the Leydig cells in the interstitial tissue of the testis to secrete testosterone. Testosterone stimulates development of the germinal epithelium (supporting sperm production) and maintains male secondary sexual characteristics; its release in turn inhibits LH and GnRH secretion by negative feedback.

The female reproductive system consists of the gonads (two ovaries) and the accessory organs: the oviducts (fallopian tubes), the uterus (made up of the fundus, body and cervix, with perimetrium, myometrium and endometrium layers), and the vagina.

If the ovulated oocyte is fertilized and implants, the tiny embryo secretes human chorionic gonadotropin (hCG), which maintains the corpus luteum instead of letting it degenerate. The corpus luteum therefore continues secreting high levels of estrogen and progesterone, which keeps the endometrium intact. As a result, menstruation is prevented (does not occur) throughout gestation, and since FSH/LH cycling is suppressed by the sustained high estrogen and progesterone, ovulation also does not occur during pregnancy.

A primary spermatocyte (2n, diploid) is formed when a spermatogonium divides; it undergoes meiosis-I to produce two secondary spermatocytes (1n, haploid). The secondary spermatocytes quickly undergo meiosis-II to form spermatids.

Spermatogenesis occurs in the seminiferous tubules of the testes, begins at puberty and continues throughout life; each spermatogonium produces four equal, functional haploid spermatids. Oogenesis occurs in the ovary, begins during embryonic development (with primary oocytes arrested in prophase-I until puberty), and each primary oocyte produces only one large functional ovum along with small, non-functional polar bodies, with normally only one oocyte maturing and being ovulated per monthly cycle.

Sertoli cells are found in the germinal epithelium (wall) of the seminiferous tubules; they nourish the developing sperms and, on FSH stimulation, release inhibin. Leydig cells are found in the interstitial tissue between the seminiferous tubules; on LH stimulation, they secrete the hormone testosterone.

Spermatids (1n) are the immature haploid cells produced directly after meiosis-II; they are rounded and non-motile. Each spermatid matures/elongates into a spermatozoon (sperm), which has a distinct head, midpiece and tail and becomes motile only after passing into the epididymis.

The primary oocyte (2n, diploid) is formed from an oogonium by mitosis during embryonic development and remains arrested in prophase-I until puberty. At puberty it completes meiosis-I to form the secondary oocyte (1n, haploid) and the first polar body; the secondary oocyte then arrests in metaphase-II and is the cell that is actually released at ovulation.

Seminiferous tubules → rete testis → epididymis → vas deferens → ejaculatory duct → urethra → outside through the penis.

Spermatogenesis is a continuous process that starts at puberty and produces millions of sperm, since each spermatogonium gives rise to four functional spermatids; this huge number is needed because only one sperm out of millions succeeds in surviving the journey through the female tract and fertilizing the ovum, so producing many sperm maximizes the chance of successful fertilization. In contrast, oogenesis is a cyclic process in which, from the limited pool of primary oocytes already present at birth, normally only one oocyte matures and is ovulated per month, because only a single fertilized egg (or a few, in case of twins) can be supported by one pregnancy at a time, and the female body invests far more resources (cytoplasm, nutrients) into each large ovum than the male invests in each tiny sperm.