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Development of male gametophyte

This document summarizes the development of the male gametophyte or microsporogenesis in plants. It begins with an introduction describing how microspores develop into a vegetative cell and generative cell. It then discusses how the generative cell undergoes mitosis to form two sperm cells. The document proceeds to describe the formation of the vegetative and generative cells from the microspore, including their sizes, shapes and contents. It concludes by discussing the development of the pollen wall, which has two layers - the outer exine and inner intine.

Education◦
DEVELOPMENT OF MALE GAMETOPHYTE PRESENTED BY, NIRALI M PARMAR, MSC.SEM-2 BOTANY CBO-404 DEPARTMENT OF LIFE SCIENCES H.N.G.U. PATAN
CONTENTS • Introduction • Microspore • formation of the vegetative & generative cell • division of the generative cell • Pollen wall
INTRODUCTION • The microspores or the pollen grains represent the first cell generation of the male gametophyte. • The further development of the male gametophyte or microsporogenesis gives rise to a large vegetative cell and a small generative cell. • The latter undergoes another mitotic division to form two gametes (sperms) either in the polllen grain or pollen tube • Along with these changes the wall synthesis is also followed.
MICROSPORE • The newly formed pollen grains have very dense cytoplasm with distinct nucleus. • Soon after its release from tetrad, it undergoes rapid increased in volume. • The nucleus of the pollen may remain in resting period varying from a few days to several weeks depending upon the species. • The pollen grains have reserve food materials in the form of carbohydrates, proteins or lipids.
Formation of vegetative & generative cell • The first division of the microspore gives rise to two unequal cells: 1. The larger one is vegetative cell, which develops into pollen tube. 2. The smaller one is the generative cell, which with the subsequent mitotic division ftwo morms ale gamets or sperms.
• Geitler (1935) observed that the metaphase spindle usually shows pronounced asymmetry, the wall ward pole being blunt and the free pole acute. • The cause of asymmetry has been attributed to differences In the time of the development of the spindle poles:- • generative pole develops more slowly than the vegetative because the smaller amount of the cytoplasm is associated with the former.
• However, the microspores of the same tetrad show same stage of development but a complete synchronization is observed in massulae and pollinia, where microspores are united together as in Mimosaceae, Asclepiadaceae and Orchidaceae.
• The generative cell is either cut off inner side or on the outer side or on the radial wall or in a corner. • The generative cell soon loses contact with the wall of the microspore and come to lie freely in the vegetative cell. • Immediately after detachment, the generative cell is spherical. • However, variation in shapes, like elliptical, lenticular, spindle shaped or sometimes vermiform have also been observed in different species. • The generative cell contains all usual cell organelles except plastids with a few exceptions. • The nucleus is smaller but contains higher amount of DNA.
• The generative cell either divides when the pollens are still within the anther or it may takes place after the pollen discharge. • The two male gametes or sperms thus formed are discrete cells and their cytoplasmic sheath persists throughout their course in the pollen tube. • The tube cell, which enclose the generative cell, will produce the pollen tube, a structure essential for sperm delivery to the egg.
Polllen wall • Pollen grains rather uniform in their wall architecture: 1. tectate exine 2. Pilate exine • A mature pollen wall shows two distinct layers- outer exine & inner intine. • The exine is composed of sporopollelin, a highly resistant material believed to be produced by the oxidative polymerization of carotenoids pigments. • The sculptured part of the exine is made up of radically oriented rod like baculae which may remain open or covered by a roof or tectum.
• The exine covers the entire pollen grain except germinal apertures ( germ pores) where it is absent or highly reduced. • In a tectate grain the void between baculae open to the outside through perforations termed microspores. • The exine is composed of an outer ektexine (sexine& nexine) and an inner endexine. • The endexine is often well developed in dicots and is absent in monocots. • The intine is composed of pecto- cellulose materials . • In some taxa, particularly Poaceae a middle layer is rich in pectic polysaccharides, termed as Z- layer, is distinguishable. • This layer is thickened at germ pore and is termed "Zwischenkorper".
REFERENCE • A textbook of botany Angiosperm -B.P.PANDEY • https://scholar.google.co.in.
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