GONADS IN VERTEBRATES
The gonads are organs that produce gametes which are sperms and ova. In males gonads are called testes and in females ovaries. The gametes are haploid germ cells whether they are spermatozoa or ova. Gonads also produce sex hormones that affect body growth and secondary sexual characters.
The testes are male gonads where sperms are formed and the male sex hormone testosterone is produced by the interstitial cells or Leydigcells, which are located in the connective tissue between the seminiferous tubules. The secretion of testosterone increases markedly at puberty and is responsible for the growth of male reproductive system and for the formation of sperms. It also stimulates the development of secondary sexual characteristics like growth of beard, deepening of the voice, broadening of shoulders, enlarged and stronger bones and muscles and determines the male sexual behavior.
Testes lie in the abdominal cavity except in mammals in which they are housed in scrotal sacs outside the body. Almost all vertebrates have two testes of similar size, although in sharks right testis may be larger and in many birds and mammals the left one may be the larger one. Cyclostomes have only a single testis in the middle of the body cavity, which is formed by the fusion of paired testes of the embryo. Hagfishes also possess a single combined ovary and testis and they are supposed to be hermaphrodite animals.
Mesentery that suspends testes in abdominal cavity is known as mesorchium. In humans testis is 5 cm long and 2.5 cm wide and produces about 30 million spermatozoa every day. In mammals abdominal testes are found in elephants, whales, dolphins, sirenia, rhinoceros, insectivores, pinnipedia and edentata.
Each testis is enclosed in a fibrous coat called tunicaalbuginea and an outer peritoneal coat called tunicavaginalis. Testis lumen is divided into 200-300 lobules, inside which highly coiled seminiferoustubules called crypts are housed, inside which spermatogenesis takes place. All seminiferous tubules join together to form a network of tubules called retetestis. Several small tubules called vasaefferentia connect rete testis with epididymis, which itself is a highly coiled tubule in which maturation of sperms takes place. Ductusepididymis continues with vas deferens which eventually opens into a sac-like seminalvesicle which secretes about 60% of the seminal fluid, the rest is secreted by prostategland. A urethra carries the sperms from seminal vesicle to the outside. Urethra is lubricated by the secretion of the bulbo-urethral glands.
All vertebrates have similar arrangement of tubules and mechanism of production and transport of sperms, with minor modifications.
One pair of ovaries lies in the abdomen in all vertebrates and they secrete hormones, Oestrogens, progesterone and relaxin. Ovaries are suspended in the abdominal cavity by the double layered peritoneal membrane called mesovarium. The germinal layer is made of cuboidal cells which produces oogonia. Underlying the germinal layer is a fibrous layer, tunicaalbuginea. The body of ovary is made of an outer tissue called cortex made of dense tissue and the inner loose connective tissue called medulla. Human ovary is 3.5 cm long and 2.0 cm wide. Ovary contains about 200,000 follicles, of which 20-25 begin to develop each month but only one is able to attain maturity, the rest undergoing atresia or death and decay.
Graafianfollicle is enclosed in an outer layer of flattened cells called thecaexterna and an inner layer of rounded cells called thecainterna. Inside the follicle there are follicular cells which serve to provide nourishment to the developing ovum. Ovum is enclosed in a gelatinous transparent envelope called zonapellucida. The ovum rests on the germ hill or discusproligerous. Cells of the mature Graafianfollicle secrete Oestrogen upon stimulation by folliclestimulatinghormone of the anterior pituitary. The luteinizinghormone secreted by the anterior pituitary causes rupture of the Graafian follicle so that ovum can be released.
As the graafian follicle is ruptured and ovum is released into coelom, a cavity is left in the ovary that is quickly filled with blood. It is called corpushaemorrhagicum, which later is filled with yellowish fluid and is therefore called corpusluteum which functions as an endocrine gland and secretes progesteron. If ovum is not fertilized then corpus luteum is reduced and becomes whitish and is therefore called corpusalbicans. The cycle is then repeated all over again.
Ovaries are hollow or saccular in fishes and amphibians but solid in amniotes. In saccular ovaries ova are shed either internally into the lumen of the ovary or to outside into the coelom, but in solid ovaries ova are always released outside into the coelom from where they are collected by the funnel-like ostiumtubae and transported to the outside.