Cholesterol is found only in animals; it is not found in plants although they can produce phytoestrogens from cholesterol-like compounds called phytosterols.

Because cholesterol cannot be dissolved in the blood, it must be carried through the body on a "carrier" known as a lipoprotein. A lipoprotein is cholesterol covered by protein. There are two types of liproproteins-LDL (low density  ipoprotein) and HDL (high density lipoprotein).

All steroid hormones are synthesized from cholesterol through a common precursor

steroid, pregnenolone, which is formed by the enzymatic cleavage of a 6-carbon side-chain

of the 27- carbon cholesterol molecule, a reaction catalyzed by the cytochrome P450 sidechain cleavage enzyme (P450scc, CYP11A1) at the mitochondria level (Figure 1a). The ovarian granulosa cells mainly secrete progesterone (P4) and estradiol (E2); ovarian theca cells predominantly synthesize androgens,and ovarian luteal cells secrete P4 (and its metabolite 20α-hydroxyprogesterone (Hu et al., 2010). Progesterone is also synthesized by the corpus luteum and by the placenta in many species as it will be mentioned later. 

Testicular Leydig cells are the site of testosterone (T) production. The brain also synthesizes steroids de novo

from cholesterol through mechanisms that are at least partly independent of peripheral steroidogenic cells. Such de novo synthesized brain steroids are commonly referred to as neurosteroids.

In mammals, the adrenal or suprarrenal glands are endocrine glands that produce at the outer adrenal cortex androgens such as androstenedione.

All these steroidogenic tissues and cells have the potential to obtain cholesterol for steroid

synthesis from at least four potential sources: 

a) cholesterol synthesized de novo from acetate;

b) cholesterol obtained from plasma low-density lipoprotein (LDL) and high-density

lipoprotein (HDL); 

c) cholesterol-derived from the hydrolysis of stored cholesterol esters in

the form of lipid droplets; 

d) cholesterol interiorized from the plasma membrane, all this mechanisms implicating cell organels such as smooth endoplasmic reticuli, endosomes and of course mitochondria (Figure 1b). 

Although all three major steroidogenic organs (adrenal, testis and ovary) can synthesize cholesterol de novo under the influence of the tropic hormone, the adrenal and ovary preferentially utilize cholesterol supplied from plasma LDL and HDL via the LDL-receptor mediated endocytic pathway.

The use of LDL or HDL as the source of cholesterol for steroidogenesis appears to be species dependent; rodents preferentially utilize the SR-BI/selective pathway; this is a process in which cholesterol is selectively absorbed while the lipoprotein (mainly HDL) remains at the

cell surface. The discovery of a specific receptor for this process (scavenger receptor class B, type I, known as SR-BI) has revolutionized the knowledge about the selective uptake pathway as a means of achieving cholesterol balance (Azhar et al., 2003).

Humans, pigs and cattle primarily employ the LDL/LDL-receptor endocytic pathway to meet their cholesterol need for steroid synthesis. In contrast, testicular Leydig cells under normal physiological conditions rely heavily on the use of endogenously synthesized cholesterol for androgen (testosterone) biosynthesis.