menstrual cycle

MENSTRUAL CYCLE

Menstrual cycle is the cyclic changes in the reproductive tract of primate (Man, Monkey and Apes) females. Menstruation is the periodic shedding of the endometrium of uterus with bleeding. In healthy women, menstruation occurs at intervals of about 28 to 29 days.

Menarche is the starting of menstruation in girls that occurs at about 13 years of her age. Menstrual cycle consists of menstrual phase, proliferative phase (follicular phase) and secretory phase (luteal phase).

Proliferative phase (5th to 14th day of menstrual cycle) consists of growth of endometrium of uterus, fallopian tube and vagina. In ovary, a Graafian follicle secretes estrogen during this phase.

Estrogen is the hormone active during proliferative phase. The ovum is released from the follicle near the end of proliferative phase, i.e., 14th day or midway during menstrual cycle.

Ovulation occurs under the influence of LH from pituitary. The subsequent 14 days in which corpus luteum is active is referred to as the secretory phase.

Progesterone secreted by corpus luteum is active during secretory phase. The uterine endometrium and glands grow further during secretory phase.

At the end of secretory phase, corpus luteum degenerates into corpus albicans in the ovary, progesterone secretion falls, the overgrown uterine endometrium breaks down and mensturation takes place.

Menstrual cycle is controlled by FSH, LH, estrogen and progesterone. The menstrual cycle and menstruation remain suspended during pregnancy and lactation. Menopause (climacteric) is the period of life when menstruation naturally stops.

Menopause occurs in females at the age of around 45-50 years. Ability to reproduce is lost in the female after menopause.

Various Phases of the Menstrual Cycle

ESTROUS CYCLE

The estrous cycle consists of cyclic changes in the female reproductive system of non-primate mammals. There is no menstruation at the end of estrous cycle.

The estrogen level in blood increases resulting strong sex urge in the female. This is called "period of heat".

The estrous cycles run only during breeding season. The estrous cycles remain suspended in female during non-breeding season. The suspension of estrous cycles is called the state of anestrum.

Those animals that have only a single estrous during the breeding season are called monoestrous, e.g., dog, fox, deer, bat, etc.

The animals that have a recurrence of estrous during breeding season are called polyestrous, e.g., Mouse, squirrel, cow, sheep, pig, horse etc.

Concept Builder

Types of Egg

The eggs are classified on the basis of amount and distribution of yolk in them. Yolk is the reserve food material in the cytoplasm of egg. It may be formed of phospholipids, proteins, or lipid and carbohydrates. The process of formation of yolk and its deposition in egg. is called vitellogenesis.

1. Based on amount of yolk.

(i) Alecithal -Eggs without yolk e.g. human egg.

(ii) Microlecithal -The eggs with very little yolk, e.g. sea urchin, starfish.

(iii) Mesolecithal -The eggs containing moderate amount of yolk, e.g. frog.

(iv) Megalecithal (or Macrolecithal) -The eggs containing very large amount of yolk, e.g. reptiles, birds.

2. Based on distribution of yolk.

On the basis of distribution of yolk, the eggs are classified as follows

(i) Isolecithal -The yolk is uniformly distributed throughout the cytoplasm of egg, e.g., Branchiostoma, Herdmania.

(ii) Telolecithal-The eggs in which the yolk is concentrated towards one pole i.e., vegetal pole & nucleus along with major part of cytoplasm is displaced is animal pole e.g. , amphibians.

(iii) Centrolecithal -Yolk concentrated in centre of the egg with cytoplasm surrounding it, e.g. insects.

Cleidoic Eggs

Eggs of reptiles and birds are insulated by albumen, membranes and shell. The calcareous shell present around the eggs of bird is mainly made up of calcium carbonate (94%) and is secreted by uterus (shell glands). Chalazae are the suspensory ligaments of the yolk in birds's egg.

Egg Membranes

There are three types of egg membrane which can be present around the egg

(i) Primary egg membrane: It is formed around the plasma membrane of egg secreted by ovum itself e.g, Jelly envelope (Echinoderms). Vitelline membrane (Mollusca, amphibians & birds), zona radiata (Shark, some amphibians).

(ii) Secondary egg membrane: It is formed around an egg by the follicle cells of the ovary e.g., chorion around egg of insect; corona radiata, granulosa & theca layers in human.

(iii) Tertiary egg membranes: These are formed by the oviducts & other accessory parts of maternal genital tract while the egg is passing from the ovary to the exterior. e.g., the albumin, shell membrane & outermost calcareous shell of reptiles & birds.

Types of Cleavage

The mode of cleavage is determined by the amount of yolk and its distribution.

(a) Holoblastic: The cleavage in which the segmentation lines pass through the entire egg , dividing it completely. It occurs in alecithal, microlecithal and mesolecithal eggs; e.g. frog, human egg etc. It is of two types equal holoblastic example Amphioxus and Marsupials; unequal holoblastic e.g., lower fishes and amphibians.

(b) Meroblastic: The lines of segmentation do not completely pass through the egg and remain confined to a part of the egg. Such type of cleavage is found in megalecithal eggs as the yolk provides resistance to the cleavage, e.g. insects, birds, reptiles. Meroblastic cleavage may be (i) Superficial which occurs in centrolecithal eggs, e.g . insects, or (ii) discoidal, found in eggs of birds.

Patterns of Cleavage

Cleavage is the successive mitotic cell divisions of the egg & can be in following pattern

(i) Radial Cleavage: Successive clevage plane cuts straight through the egg e.g. Synapta paracen rotus.

(ii) Biradial Cleavage: When the first three division planes do not stand at right angles to each other: Ctenophora

(iii) Spiral Cleavage: There is a rotational movement of cell parts around the egg axis leading to displacement of mitotic spindle with respect to symmetrically disposed radii e.g. turbellarians, nematoda, rotifera, annelida, all mollusc except cephalopods.

(iv) Bilateral Cleavage: Mitotic spindles and cleavage planes remain bilaterally arranged with reference to the plane of symmetery e.g. tunicates, Amphioxous, amphibia and higher mammals.

(v) Meridional Cleavage: When cleavage furrow bisects both the poles of egg passing through animal vegetal axis, the plane of cleavage is called meridional cleavage.

According to the concept of potency, cleavage can be

(i) Indeterminate/Regulative eggs, the fate of different parts of egg or its blastomeres is not predetermined or blastomeres have no characterstic position & alterable fate e.g. Echinoderms and vertebrates.

(ii) Determinate/Mosaic eggs. Every part of fertilized egg has a definite fate, so that fate of every blastomere is determined from beginning i.e. , produces the complete embryo only if all the blastomeres remain together. e.g., Nematoda, Mollusca (Dentalium).

Morula

Cleavage results in the formation of solid mass of cells called Morula as it looks like a mulberry.

Blastula

Blastula is the embryonic stage next to morula.

It contains a fluid-filled cavity called blastocoel surrounded by one or more layers of cells, the blastomeres.

The blastula with blastocoel is called coeloblastula, e.g. frog.

In certain animals, the blastula is solid and is termed stereoblastula e.g., Cnidaria, Nereis & some molluscs.

The blastula formed as a result of superficial cleavage is called superficial blastula, e.g. insects.

Discoblastula is disc shaped blastula formed as a result of discoidal cleavage, e.g. birds.

Gastrula

Gastrula is formed by gastrulation. Gastrulation is the process of the formation of gastrula from the blastula. Gastrula is the embryonic stage of development in which the germinal layers are formed. Gastrulation involves movement of cells.

The movements are called morphogenic movements. They include epiboly (descending of dividing cells to cover other cells) , emboly (upward movement of dividing cells underneath the other cells), involution (inward migration of blastomeres to go into the blastocoel), invagination (tucking in of blastula wall) , and delamination (separation of a sheet of cells from blastula) .

Archenteron is the cavity which occurs inside the gastrula, and is the future alimentary canal. It opens to the outside through blastopore which closes later.

Foetal Membranes (Extra embryonic membranes)

The developing foetus is enclosed by three membranes, amnion, chorion and allantois.

(i) Amnion is formed from mesoderm outside and ectoderm inside. Space between amnion and foetus is called amniotic cavity and it contains amniotic fluid.

(ii) Chorion is formed from ectoderm externally and mesoderm inside. Alongwith the allantois it participates in the formation of placenta. Space between amnion and chorion is embryonic coelom .

(iii) Allantois consists of mesoderm outside, and endoderm inside. It extends to fuse with chorion and forms allanto-chorion which gives rise to foetal part of placenta.