Page 338 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

320 Veterinary Histology of Domestic Mammals and Birds

9, ‘Endocrine System’). Neurosecretory cells located in In the oestrous phase, the follicle develops to full
VetBooks.ir hypothalamic nuclei periodically secrete releasing and inhib- maturity and blood oestrogen levels peak. Levels of LH
iting hormones that are transported by the hypophyseal rise rapidly while FSH secretion begins to decline. In most
portal system to the adenohypophysis, where they stimulate species, a surge in LH is followed by ovulation (spontane-
or inhibit the release of gonadotrophic hormones. ous ovulation). Ovulation in cats occurs in response to
Ovarian function is regulated by the gonadotropins mating (induced ovulation). Depending on species, ovula-
follicle-stimulating hormone (FSH), luteinising hormone tion occurs at the end of oestrus or at various timepoints
(LH) and prolactin. Luteinising hormone stimulates the during metoestrus.
production of androgens (testosterone, androstenedione) In metoestrus, the corpus haemorrhagicum trans-
by thecal cells. Follicle-stimulating hormone activates the forms into the corpus luteum, which begins to synthesise
aromatase enzyme system of the granulosa cells for con- progesterone.
version of androgens to oestrogens. The ratio of FSH:LH The corpus luteum reaches maximum activity in dioes-
determines the timing of ovulation. Production of pro- trus. Regression of the corpus luteum is triggered by
gesterone by the corpus luteum is mediated by prolactin. prostaglandins (PGF ) produced by the uterine mucosa
2α
Feedback loops regulate the release of gonadotropins from (corpus luteum cyclicum). If conception occurs, progester-
the adenohypophysis. one production is maintained (corpus luteum graviditatis)
The oestrous cycle is divided into the following phases: and prostaglandin-induced retrogression of the corpus
luteum is delayed until the end of pregnancy. Regression
· pro-oestrus, is followed by development of the corpus albicans.
· oestrus,
· metoestrus and Hormonal effects on the uterine tubes
· dioestrus. The uterine tubes are quiescent in pro-oestrus. During
oestrus, rising oestrogen levels lead to increased activity
In theriogenology, metoestrus, dioestrus and pro-oestrus of the secretory epithelial cells.
are also combined under the term interoestrus, repre- The greatest volumes of secretion are produced during
senting the period between oestrous phases of the cycle. metoestrus when the oocyte is in the ampulla tubae uteri-
Anoestrus, the period in which oestrus does not occur, is nae. This is accompanied by increased beating of the cilia.
observed in monoestrous species (bitch, see below) at the In the subsequent phase, dioestrus, the activity of the
end of a prolonged metoestrus, or after conception. uterine tube epithelium rapidly declines. The cells become
Division of the oestrous cycle into phases is based on shorter and partly degenerate.
structural changes in the reproductive organs and associ-
ated fluctuations in circulating hormone concentrations. Cyclic changes in the uterus
Cyclic changes in FSH, oestrogens, progesterone, LH and During pro-oestrus, the uterine epithelium grows, the
prolactin vary among species. Hormone levels measured glands enlarge but remain straight and vascularisation
in the blood do not necessarily indicate the sensitivity of increases (proliferative phase).
hormone receptors on the target cells (refer to veterinary Uterine changes reach their peak during oestrus. Under
endocrinology and physiology texts). the influence of oestrogen, epithelial cells and uterine
The length of the oestrous cycle varies considerably glands release secretions (uterine milk), the spinocellular
among domestic mammals. Based on cycle frequency, a subepithelial connective tissue thickens and intercellular
distinction is made between: spaces become expanded and oedematous.
Hyperplasia of the uterine glands continues into
· polyoestrous species (e.g. mare, cow, sow) and metoestrus. During this phase, the connective tissue layer
· monoestrous species (e.g. bitch). thins and oedema decreases.
In dioestrus, the glands become extensively coiled and
In monoestrous animals, an extended period of metoes- shortened. Progesterone brings about maximal glandu-
trus (50–70 days) is followed by anoestrus. In polyoestrous lar secretion, which is sustained if pregnancy occurs. If
species, the corpus luteum regresses and the animal enters implantation does not take place, secretory activity gradu-

pro-oestrus and then oestrus (see endocrinology and ally decreases, the blood supply decreases and the glands
theriogenology texts). revert to their tubular morphology (involution).
Changes in the cervix are closely linked with those
Cyclic changes in the ovary occurring in the uterus. Under the influence of oestrogens,
During pro-oestrus, release of FSH from the adenohy- the cervical epithelium secretes increasing amounts of
pophysis brings about follicular maturation. Oestrogen thin, clear mucus. During dioestrus, the mucus becomes
secretion increases. more viscous.

Vet Histology.indb 320 16/07/2019 15:05