like oestradiol, gen istein, occurs in subterranean clover, and this, by upsetting the
normal levels in foraging sheep, brings about infertility in the ewes.
A variety of steroids have been identified in the blood and certain tissues of
vertebrates, and some of these are specific hormones. Others probably represent
products and by-products in the metabolic synthesis and degradation of the hormones, and also result from the chemical insults perpetrated on the tissues by the
investigator. The vertebrate steroid hormones can be conveniently classified into
three main groups chemically, or four physiologically. Chemically this is based on
the number of carbon atoms (C 21,C19and C 18 ) and physiologically (in italics) on the
general spectrum of their action and origin. 1. C 2 1 steroids. These include ; the adrenocortical (or interrenal) steroids, principally cortisol (hydrocortisone), corticosterone and aldosterone, and the progestogens, including progesterone, which
are secreted mainly by the ovary. 2. C I9 steroids include: the androgens like testosterone from the gonads of the male animal. 3. C I 8 steroids: the oestrogens, like
oestradiol, which are secreted by the ovary. This list is not exhaustive, but is relevant to our subject. The chemical structures of the principal examples from each
group are given in Table 2.3. It is interesting to see that relatively small changes
in the chemical groups of the steroid nucleus confer widely different physiological
activities on the molecules. It is, nevertheless, not surprising that each substance
is not completely specific in its actions, a small mutual cross-over in biological activity often being apparent.
7. The Gonads
Oestrogens, progestogens and androgens have been identified in various species
throughout the vertebrate series (see NANDI, 1967), though among the lower vertebrates demonstrations of their presence in the circulation are sparse.
Oestrogens are secreted by the follicular tissue of the ovary, and have also been
found in the mammalian placenta, the testis and adrenocortical tissue. In the female
animal the oestrogens integrate the processes of growth, differentiation, maturation
and maintenance of the organs and tissues concerned with reproduction. That
ethereal entity, female 'behaviour,' is also largely due to the actions of the oesrrogens . Oestradiol is the most potent of the naturally occurring oestrogens, and is
usually considered to be the principal and 'parent' hormone. Other oestrogens
also widely found, include oestriol and oestrone which are probably derived from
the ' parent' compound as a result of its metabolic transformation.
Progestogens, of which the principal natural example is progesterone, are
formed by the ovary of a wide variety of vertebrates, and have also been found
in adrenocortical tissue and in the mammalian placenta. Progesterone is necessary
for the maintenance of pregnancy and the growth of the secretory alveoli of the
mammary glands. Its function in the lower vertebrates is unknown, but it may be
concerned with gestation in viviparous and oviviviparous species of the Reptilia,
the Amphibia and the Chondrichthyes. Progesterone ma y be a metabolic intermediate in the biogenesis of all steroid homones (GRANT, 1962), so that its presence
in tissues that form steroid hormones need not imply a direct endocrine function .
66
normal levels in foraging sheep, brings about infertility in the ewes.
A variety of steroids have been identified in the blood and certain tissues of
vertebrates, and some of these are specific hormones. Others probably represent
products and by-products in the metabolic synthesis and degradation of the hormones, and also result from the chemical insults perpetrated on the tissues by the
investigator. The vertebrate steroid hormones can be conveniently classified into
three main groups chemically, or four physiologically. Chemically this is based on
the number of carbon atoms (C 21,C19and C 18 ) and physiologically (in italics) on the
general spectrum of their action and origin. 1. C 2 1 steroids. These include ; the adrenocortical (or interrenal) steroids, principally cortisol (hydrocortisone), corticosterone and aldosterone, and the progestogens, including progesterone, which
are secreted mainly by the ovary. 2. C I9 steroids include: the androgens like testosterone from the gonads of the male animal. 3. C I 8 steroids: the oestrogens, like
oestradiol, which are secreted by the ovary. This list is not exhaustive, but is relevant to our subject. The chemical structures of the principal examples from each
group are given in Table 2.3. It is interesting to see that relatively small changes
in the chemical groups of the steroid nucleus confer widely different physiological
activities on the molecules. It is, nevertheless, not surprising that each substance
is not completely specific in its actions, a small mutual cross-over in biological activity often being apparent.
7. The Gonads
Oestrogens, progestogens and androgens have been identified in various species
throughout the vertebrate series (see NANDI, 1967), though among the lower vertebrates demonstrations of their presence in the circulation are sparse.
Oestrogens are secreted by the follicular tissue of the ovary, and have also been
found in the mammalian placenta, the testis and adrenocortical tissue. In the female
animal the oestrogens integrate the processes of growth, differentiation, maturation
and maintenance of the organs and tissues concerned with reproduction. That
ethereal entity, female 'behaviour,' is also largely due to the actions of the oesrrogens . Oestradiol is the most potent of the naturally occurring oestrogens, and is
usually considered to be the principal and 'parent' hormone. Other oestrogens
also widely found, include oestriol and oestrone which are probably derived from
the ' parent' compound as a result of its metabolic transformation.
Progestogens, of which the principal natural example is progesterone, are
formed by the ovary of a wide variety of vertebrates, and have also been found
in adrenocortical tissue and in the mammalian placenta. Progesterone is necessary
for the maintenance of pregnancy and the growth of the secretory alveoli of the
mammary glands. Its function in the lower vertebrates is unknown, but it may be
concerned with gestation in viviparous and oviviviparous species of the Reptilia,
the Amphibia and the Chondrichthyes. Progesterone ma y be a metabolic intermediate in the biogenesis of all steroid homones (GRANT, 1962), so that its presence
in tissues that form steroid hormones need not imply a direct endocrine function .
66