(a) Draw a well-labelled diagram of testis and ovary in mammals, and discuss the functions of their hormones. 20 (b) Give an…

(a) Mammalian testis and ovary and the functions of their hormones

The testis is enclosed by a tough fibrous capsule, the tunica albuginea, from which septa divide it into lobules. Each lobule contains coiled seminiferous tubules lined by germinal epithelium, in which spermatogonia, spermatocytes, spermatids and spermatozoa lie between tall Sertoli cells. Sertoli cells, joined by tight junctions, form the blood–testis barrier, nourish developing sperm, and secrete androgen-binding protein, inhibin and anti-Müllerian hormone. Interstitial Leydig cells lie in the connective tissue between the tubules, close to blood vessels, and secrete testosterone. The tubules drain through the rete testis and efferent ductules into the epididymis.

The ovary has an outer germinal epithelium, a tunica albuginea, a cortex containing follicles, and a central medulla with blood vessels. The cortex shows follicles at every stage: primordial follicles (an oocyte with flat follicular cells), primary follicles (cuboidal granulosa cells and a zona pellucida), secondary follicles (several granulosa layers with theca interna and theca externa), and the mature Graafian follicle with a fluid-filled antrum and the oocyte on a cumulus oophorus. After ovulation the follicle becomes the corpus luteum, which later regresses into the corpus albicans; atretic follicles are also seen.

Both gonads are controlled by the hypothalamic–pituitary–gonadal axis. GnRH from the hypothalamus stimulates the anterior pituitary to release FSH and LH.

• Testosterone (from Leydig cells, under LH): with FSH acting on Sertoli cells, it maintains spermatogenesis; it develops and maintains the male accessory glands and secondary sex characters (voice, hair, muscle mass), promotes libido and protein anabolism, and during foetal life masculinises the reproductive tract. It is converted to dihydrotestosterone (external genitalia, prostate) and to oestradiol (bone, brain), and gives negative feedback to the hypothalamus and pituitary.
• Inhibin (from Sertoli cells): selectively suppresses FSH secretion.
• Anti-Müllerian hormone (from foetal Sertoli cells): causes regression of the Müllerian ducts in male embryos.
• Oestrogens (from granulosa cells): LH-stimulated theca cells make androgens that FSH-stimulated granulosa cells aromatise to oestradiol. Oestradiol causes endometrial proliferation, growth of the ducts of the mammary glands and female secondary sex characters; at low levels it gives negative feedback, but a sustained high level triggers the pre-ovulatory LH surge by positive feedback.
• Progesterone (from the corpus luteum, later the placenta): makes the endometrium secretory for implantation, maintains pregnancy by quietening the myometrium, and develops the mammary alveoli.
• Inhibin, activin and relaxin: ovarian inhibin suppresses FSH; relaxin from the corpus luteum and placenta softens the cervix and pubic symphysis before birth.

In many mammals, prostaglandin F2α from the uterus causes luteolysis and ends the oestrous cycle, whereas menstruation occurs mainly in primates. Thus both sexes share the same axis but differ in cyclicity, target organs and the fate of the corpus luteum.

(b) Skull types in reptiles

Reptile skulls are classified by the number and position of temporal fenestrae, openings behind the orbit in the dermal roof of the skull. Fenestrae lighten the skull and give room and attachment edges for the bulging adductor jaw muscles. The four classical types are:

• Anapsid: no temporal opening; the roof behind the orbit is solid. It occurs in early reptiles such as captorhinids and pareiasaurs and in turtles and tortoises (Testudines). Molecular and fossil evidence now suggests that turtles are diapsids that secondarily closed their fenestrae.
• Synapsid: a single lower temporal fenestra bounded above by the postorbital and squamosal. It characterises pelycosaurs and therapsids, the lineage leading to mammals; these are now regarded as a separate clade, Synapsida, rather than true reptiles.
• Euryapsid (parapsid): a single upper temporal fenestra bounded below by the postorbital and squamosal, as in the extinct marine ichthyosaurs and plesiosaurs; it is thought to be derived from the diapsid condition by loss of the lower opening.
• Diapsid: two temporal fenestrae on each side, separated by a bar formed by the postorbital and squamosal. The upper fenestra is bounded mainly by the parietal, postorbital and squamosal; the lower by the postorbital, jugal, quadratojugal and squamosal. It occurs in Sphenodon, crocodilians, dinosaurs and (in modified form) birds, lizards and snakes.

Within diapsids the arches are often modified. Sphenodon and crocodilians keep both arches (crocodilians also have an antorbital fenestra). Lizards have lost the lower temporal bar, freeing the quadrate (streptostyly), and snakes have lost both bars, giving a highly kinetic skull with loosely joined jaws for swallowing large prey. Birds have lost the postorbital bar, and their prokinetic or rhynchokinetic skulls allow the upper beak to move.

(c) Types and significance of migration in fishes

Migration is a regular, directed, usually seasonal mass movement of fish between habitats, followed by return, in relation to breeding, feeding or refuge. The main types are:

• Anadromous (diadromous): live in the sea and ascend rivers to spawn, such as Pacific and Atlantic salmon, sturgeons and the hilsa (Tenualosa ilisha), which moves from the Bay of Bengal into the Ganga, Padma and Hooghly during the monsoon.
• Catadromous (diadromous): live in fresh water and descend to the sea to spawn, such as freshwater eels (Anguilla), which breed in the Sargasso Sea (Atlantic species) or the western Pacific, their leptocephalus larvae drifting back to the coasts.
• Amphidromous (diadromous): move between sea and fresh water for feeding rather than breeding, as in many gobies.
• Potamodromous: migrate wholly within fresh water, such as the Indian major carps (catla, rohu, mrigal) moving to flooded shallows to spawn with the monsoon, and mahseer (Tor) moving upstream in hill streams.
• Oceanodromous: migrate within the sea, such as tunas, the oil sardine (Sardinella longiceps) and the Indian mackerel (Rastrelliger kanagurta) along the Indian coasts, following plankton and upwelling.
• Other movements: vertical diel migration in the water column, and latitudinal or contranatant/denatant movements with and against currents.

Cues and mechanisms: day length, temperature, rainfall and floods, salinity and lunar cycles trigger migration. Hormones prepare the fish: thyroid hormones, cortisol and growth hormone drive smoltification and seawater tolerance in salmon, prolactin favours freshwater adaptation, and gonadotrophins and sex steroids bring gonadal maturity. Navigation uses olfactory imprinting on the home stream (salmon), geomagnetic fields, the sun, currents and temperature gradients.

Significance:

• Biological: it brings adults to favourable spawning and nursery grounds, reduces competition between young and adults, exploits seasonal food and helps escape unfavourable conditions.
• Ecological: salmon carry marine nutrients far inland, and migrations link river, estuary and sea ecosystems.
• Economic: migrations create seasonal fisheries such as the hilsa and oil-sardine fisheries, and spawning runs of carps supply wild seed for aquaculture.
• Conservation: dams and barrages block runs (for example, the Farakka barrage reduced hilsa migration upstream), so fish passes, environmental flows and closed seasons are needed; India enforces a uniform 61-day monsoon fishing ban on each coast, and the American and European eels are threatened by barriers and overfishing.