Structure of human sperm and ovum

A brief account of different parts of sperm and its constitutions and,ovum parts and different envelops.all things are explained by a simple attractive diagram.

1. STRUCTURE OF HUMAN SPERM AND OVUM

2. STRUCTURE OF SPERM A spermatozoan is a haploid male gamete whose primary function is to fuse with ovum to restore diploid and to transmit paternal characters to the offspring’s. A mammalian sperm is minute, microscopic, flagellated and motile gamete with no nutritive material, protective envelopes and most of cell- organelles like ribosome, endoplasmic reticulum, etc. The whole body of sperm is enveloped by plasma membrane only. Structure; basically it is consists of four regions, namely head, neck, middle piece and tail. Head: head is consists of an acrosome and a nucleous . Acrosome (Gr. akron = extremity; soma=body): It is small cap-like pointed structure present at the tip of nucleus. It is formed from a part of Golgi body of spermatid. During the sperm entry, the acrosome secretes a lytic enzyme, called hyaluronidase, which helps in the penetration of ovum. Nucleus: It is generally long, narrow and pointed but is flat and oval in human sperm. It is formed by condensation of nuclear chromatin of spermatid and loss of RNA, nucleolus and acidic proteins. Chemically, the nucleus is formed of deoxyribonucleoprotein (DNA + basic proteins). It is the carrier of genetic information. 2. Neck: It is the smallest part of spermatozoan and may be indistinct. It is formed of two centrioles perpendicular to each other and is formed from the centrosome of spermatid. Each centriole is a micro tubular triplet structure having 9 + 0 arrangement. Proximal centriole lies in a depression in the posterior surface of the nucleus and is perpendicular to main axis of the sperm. Distal centriole is along longitudinal axis of the sperm. Centrioles form spindle for the first cleavage of zygote. Distal centriole acts as basal body and gives rise to axoneme of the sperm-tail .

3. 3. Middle piece: It lies behind the neck and is cylindrical in the human sperm. It is formed of a mitochondrial spiral, nebenkem , around the proximal part of axoneme. The mitochondria are the carriers of the oxidative enzymes and the enzymes which are responsible for oxidative phosphorylation. So the middle piece is the powerhouse of a sperm. Posterior half of nucleus, neck and middle piece of sperm are covered by a sheath, manchette. 4. Tail (flagellum): It is the longest part of sperm. It is slender and tapering part. It is formed of two parts. Central, contractile and micro tubular part called axoneme or axial filament, and outer protoplasmic sheath. Axoneme is formed of 11 proteinous microtubules arranged in 9 + 2 manner. Sometimes, a ring centriole may be present at the junction of middle piece and flagellum. Tail shows lashing movements which provide forward push to the sperm. Sometimes, the distal part of axoneme is uncovered and is called end piece. Viability: It is the period up to which the sperm is able to fertilize an ovum. Viability of human sperm is about 24 hours.

4. ovum Definition: Ovum is a maternal haploid gamete and is primarily concerned with receiving the sperm and is determined to develop into a fully developed multicellular organism after fertilization (syngamy) or without fertilization (parthenogenesis). It is generally with reserve food and is genetically programmed. The egg is the female gamete,microlecithal,microscopic , spherical. Human ovum is microlecithal with large amount of cytoplasm. Cytoplasm is differentiated into outer, smaller and transparent exoplasm or egg cortex and inner, larger and opaque endoplasm or ooplasm. Egg cortex is with some cytoskeletal structures like microtubules and microfilaments ( Balinsky , 1981), pigment granules and cortical granules of mucopolysaccharides. Endoplasm is with cell-organelles, informosomes , tRNAs, histones, enzymes etc. Nucleus of ovum is large, bloated with nucleoplasm and is called germinal vesicle. Nucleus is excentric in position so human ovum has a polarity. The side of ovum with nucleus and polar body is called animal pole, while the opposite side is called vegetal pole. Egg envelopes: Human ovum is surrounded by a number of egg envelopes: 1. Corona radiate: It is outer, thicker coat formed of radially elongated follicular cells. Between the vitelline membrane and zona pellucid there is a narrow perivitelline space Corona radiata is formed of follicle cells.

5. 2. Zona pellucid: It is middle, thick, transparent and non-cellular. It is secreted partly by follicular cells and partly by the oocyte. 1. Vitelline membrane: It is inner, thin, transparent and is secreted by ovum itself. The egg carries the chromosomes from the female parent.