Download Puberty and the oestrus cycle

Puberty and the oestrus cycle:
The endocrine and nervous systems play interwoven role in the cascade on events leading to the
formation of mature gametes, fertilization, establishment and maintenance of pregnancy, birth and,
finally, rearing of offspring. These processes begin at puberty.
In the female, puberty is marked by the onset of regular cyclic activity in the ovary affecting behavior
and the entire genital system(the oestrus cycle in domestic animals).
Factors influencing the onset of puberty include age, body weight, breed nutrition, disease and, in
some species, season of the year and proximity of a male. The ages at which the common domestic
species reach puberty are listed in Table 3-1 page 27.
After the onset of puberty, signals provided by certain regions in the brain, including the pineal
gland, hypothalamus and the pituitary gland, allow for the fertilizable oocytes.
From the anterior pituitary gland, the gonadotropins (i.e hormones stimulating cells within the
gonads) FSH (follicle-stimulating hormones) and LH (Luteinizing hormone) are released. This release
is controlled by GnRHs (gonadotropin-releasing hormones) that are secreted from the hypothalamus
and conveyed to the anterior pituitary gland through hypothalamo-hypophyseal portal blood
circulation. Secretion of GnRHs, and thus of FSH and LH is influenced by visual, olfacrory, auditory
and tactile stimuli from the environment and also by homeostatic feedback systems within the
animal.
FSH released from the pituitary gland reaches the ovary by the systemic circulation. In the ovary, it
stimulates a pool of growing follicles to develop. Follicular growth occurs when follicles are recruited
from the primordial pool and develop into primary, secondary and tertiary follicles. The majority of
growing folliclesail to complete their growth phase, most degenerate through a process referred to
as atresia with only minority completing their growth to the point of ovulation.
At least in the large domestic species follicular growth is initiated during fetal life. However, none of
the oocytes enclosed in the follicles resume meiosis until puberty is reached.
Up on activation of the primordial follicle, the follicular cells start to proliferate and form a cuboidal
monolayer around the oocyte to establish the primaryfollicle.(Figures 4-8, 4-9, 4-10,in the text
book). The follicular cells are now called granulosa cells. With this activation, a phase of oocyte
growth is initiated in which the oocyte of the domestic species grows from less than 30 µm to more
than 120µm in diameter. Granulosa cells proliferate to form several layers around the oocyte in
what becomes known as the secondary follicle. The oocyte and the surrounding granulosa cells
synthesize certain glycoproteins that are deposited between itself and the surrounding granulosa
cells as the zona pellucida. This structure is traversed by numerous projections from the innermost
granulosa cells which thereby maintain contact with the oocyte through gap junction.
The stromal cells surrounding the granulosa cells differentiate into an inner theca interna, a layer of
steroid-producing cells, and the outer theca externa, made up of concentric layers of cells that have
supportive function.
As development continues, fluid-filled spaces appear between the granulosa cells and coalesce into a
single cavity, the antrum, characterizing the tertiary follicle. In parallel with the expansion of the
antrum, the oocyte becomes located in a protrusion of granulosa cell, the cumulus oophorus,
extending into the antrum.
As the follicle develops, so does the oocyte until it achieves it’s a characteristic structure. The process
of ovulation is triggered by the preovulatory LH surge.
Follicular and oocyte maturation
The tertiary follicle continues its development and, it selected for ovulation, enters a final phase of
follicular and oocyte maturation stimulated by thepreovulatory LH surge.
The period from the onset of the LH surge to ovulation is species-specific and varies from less than
12 hr to more than 40 hr. The preovulatory maturation of the oocyte has nuclear as well as
cytoplasmic components. Nuclear maturation refers to the process of meiosis which is resumed from
diplotene stage (resting stage) of 1stmeiosis and continues to the metaphase of 2nd meiosis when the
oocyte is ovulated. The nucleus of the primary oocyte is often referred to as the germinal vesicle,
and resumption of meiosis is morphologically evidenced by breakdown of this structure.
Cytoplasmic oocyte maturation involves the structuring and modulation of many of the organelles of
the oocyte.
Oestrogens which are produced in the granulosa and thecal cells lining the follicle and surrounding
the oocyte, exert a positive feedback on the hypothalamic GnRH secretion. Another principal effect
of the oestrogens is to induce oestrous symptoms, and so puberty is often signalled by the
occurrence of the first period of oestrus or heat.(sexual receptivity).
After puberty the animal enters a phase of life characterized by repeated oestous cycles. The
oestrous cycle is subdivided into prooestrus, oestrus, metoestrus, and dioestrus (Tables 3-2,3-3)
pages 27, 28.
In most species, the increasing levels of oestrogens produced during prooestrus reach a certain
threshold level that leads to a major surge of GnRH being secreted by the hypothalamus during
oestrus. On reaching the anterior pituitary gland, the GnRH stimulates a secretory surge of mainly
LH. In the ovary, LH is needed to bring about the final maturation of the oocyte(s) and their release
through the process of ovulation.
Following ovulation,the cells that lined the follicle before its rupture become transformed into a
corpus luteum, under the continued influence of LH, during metoestrus. The development of the
corpus luteum begins gradually a few hours before ovulation and is marked by the synthesis of
progesterone, instead of oestrogens. The corpus luteum formed by these cells after ovulation is a
well-defined, sometimes slightly cavitated, spherical structure, named for its yellowish colour.
During dioestrus, the production of progesterone by the corpus luteum reaches its maximum. The
principal roles of progesterone are: (1)to exert a negative feedback on the hypothalamus,
inhibitingGnRH release and, therefore, new recruitment of oocytes for ovulation. (2) to prepare the
endometrium for pregnancy.
If insemination and conception occur, progesterone is the principal hormone responsible for the
maintenance of pregnancy.
In the non pregnant animal, the life span of the corpus luteum is relatively short, in the absence of an
embryo within the uterus, the endometrium releases prostaglandins-F2α leading to luteolysis
(regression of corpus luteum). The consequent decline in progesterone results in removal of the
progesterone block on the hypothalamic GnRH secretion and allows for resumption of oestrus cycle.
In the pregnant uterus, endometrial prostaglandin F2α release into the blood stream is blocked,
leading to persistence of the corpus luteum.
The domestic breeds of pigs and cattle are non-seasonal polycyclic animals, meaning that sows and
cows experience recurring cyclic activity throughout the year, interrupted only by pregnancy,
lactation or pathological conditions.
In contrast, the mare, ewe, doe and queen are seasonally polycyclic, their cyclicity is profoundly
influenced by the amount and timing of light. The bitch is monocyclic and experiences long of
anoestrus between single periods of oestrus. Normally, one or two (sometimes three) oestrus
periods are seen per year separated by longer anoestrus periods with no obvious seasonality.
The corpus luteum produce progesterone until the end of the third month. Thereafter, the placenta
takes over until term.