Biology Today — May 2017

Cortical

granules

Nucleus

Polar bodies

Cytoplasm

Plasma membrane

Zona pellucida

Ovum

Cells of the

corona radiata

Structure of Ovum

• Human ovum is spherical in shape.

• It is alecithal, i.e., free of yolk.

• Cytoplasm is called ooplasm.

• Nucleus is large with prominent nucleolus, and is called germinal vesicle.

• The cytoplasm is enveloped by plasma membrane, below which are present small
cortical granules. To its outerside is present a narrow perivitelline space.

• A thick noncellular zona pellucida is present outer to perivitelline space and outer
to this a very thick cellular corona radiata is present.

• The side of ovum extruding polar bodies is called animal pole while the opposite
side is called vegetal pole.

Primary follicle

Primordial follicles

Germinal epithelium

Mesovarian

Blood vessels

Corpus albicans

Regressing corpus

luteum

Mature corpus

luteum

Basal

lamina

Granulosa

cells

Secondary follicle (early)

Granulosa cells

Theca

Graafian follicle

(secondary follicle-mature)

Theca externa

Theca interna

Granulosa cells

Released ovum

Atretic follicle

Young

corpus

luteum

Blood clot

(corpus

haemorrhagicum)

Ruptured follicle

Antrum with estrogen rich

follicular fluid

Zona pellucida

Oocyte Theca Oocyte with nucleus

Fig.: Cross-section of ovary showing stages of follicular growth

Fig.: Structure of ovum

•    Oocyte shows certain structural and cellular changes while differentiating into an ovum. Such as:

•    Nuclear changes – Production of large amount of nuclear sap resulting in enlarged nucleus. Nucleolus also

increases in its size.

•    Cytoplasmic changes – Mitochondria increase in number during growth of the oocyte.

• Golgi bodies either disappear or are converted to some other structures.


• ER are devoid of ribosomes but their membranes remain perforated by pores.


• Formation of cortical granules, these are spherical bodies containing mucopolysaccharides,
  bound by a simple membrane.

Differentiation of Oocyte into Ovum

MENSTRUAL CYCLE

• Menstruation is the bleeding from the uterus of adult females at an average interval of 28/29 days. The cycle of events
starting from one menstruation till the next one is called menstrual cycle.

• The first menstruation begins at puberty and is called menarche.

• The menstrual cycle comprises of four phases: menstrual, follicular, ovulatory and luteal phase.