Introduction are derivatives of cholesterol which are synthesized by

Introduction

 

Steroidogenesis involves the synthesis of steroid
hormones that are derivatives of cholesterol which are synthesized by various
tissues, most prominently the adrenal gland and gonads. These are usually found in
chordates and arthropods. Fishes, for example
teleosts, produce several types of bioactive gonadal steroids, including
progestogens, estrogens, androgens and various derivatives of steroids.
Steroids are required for development, maintenance, homeostasis and
reproduction. Steroids direct the development of germ cells and accessory glands and
organs, as well as the modification of the behaviour, to ensure that sexual
reproduction can take place. In adult vertebrates, these steroids are produced at appropriate times
in specialized steroid producing cells called gonads. These cells express a group of
steroidogenic enzyme genes whose products modify its cholesterol and
derivatives 1. Although many steroids are identical
chemically in all major vertebrate classes, the role of these steroids may
differ. However steroid hormones have evolved in some vertebrate classes,
especially amongst fishes, to fulfil particular functions.

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Steroid
hormones are produced by steroidogenic cells of the ovary testis and brain that are required for normal reproductive function and bodily homeostasis.
Steroidogenic endocrine tissues such as the adrenal and the gonads respond to
trophic hormones and other external stimuli with rapid surge in steroid hormone
production 2. The
acute and chronic regulation of steroidogenesis is controlled by trophic
hormones that normally occur on
order of minutes and hours, respectively. Chronic regulation of steroidogenesis
by LH or ACTH occurs at the level of gene transcription 3. Cholesterol
is metabolized to pregnenolone by  cytochrome
P450 cholesterol side chain cleavage
enzyme (p450scc) and transferred from the outer to the inner
mitochondrial membrane.The Steroidogenic acute regulatory (StAR) protein is the
one which regulates the true
rate-limiting step in steroid biosynthesis, i.e. the delivery of cholesterol
from the outer to the inner mitochondrial membrane 4. The central role of StAR was proven by two
observations by robust steroid hormone synthesis followed co-transfection of
StAR and the cholesterol side-chain cleavage system into nonsteroidogenic COS-1
cells 4, 5. In other patients with StAR mutations have
congenital lipoid adrenal hyperplasia, whereby all adrenal and gonadal
steroidogenesis was distrupted 5, 6. Sex differentiation is initiated and controlled by gonadal steroid
hormones. These hormones performed
different functions and permanently differentiated into sex organs during
development. The expression of this protein is predominantly regulated
by cAMP-dependent mechanism in the adrenal and gonads.

Gonadal development

Reproduction in vertebrates depends on function  two distinct gametes, sperm and eggs that
develop into different organs, the testis and the ovary. The two reproductive organs are grossly different,
but they both are composed of developmentally common cell lineages, supporting
cells, interstitial cells and germ cells. These  are
essential for gametogenesis. A mature ovary consists an ovarian cavity,
germinal epithelium and stromal compartment. In fishes such as teleosts, the
germ line stem cells are mitotically active oogonia that reside in germinal
epithelium. Their structure is similar to surface epithelium in mammals.
Steroid hormones are produced by the follicles that are present in the stromal
compartment where the oocytes grow. In the
testis, spermatogenesis from germ line stem cells to sperm occurs in tubules or
lobules, and the interstitial tissue that produce steroid hormones resides
between these structures.

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