A germ cell is any biological cell that gives rise to the gametes of an organism
that reproduces sexually. In many animals, the germ cells originate near the gut
of an embryo and migrate to the developing gonads. There, they undergo cell
division of two types, mitosis and meiosis, followed by cellular differentiation
into mature gametes, either eggs or sperm.
Multicellular eukaryotes are made of two fundamental cell types. Germ cells
produce gametes and are the only cells that can undergo meiosis as well as
mitosis. These cells are sometimes said to be immortal because they are the link
between generations. Somatic cells are all the other cells that form the building
blocks of the body and they only divide by mitosis. The lineage of germ cells is
called germ line. Germ cell specification begins during cleavage in many animals
or in the epiblast during gastrulation in birds and mammals. After transport,
involving passive movements and active migration, germ cells arrive at the
developing gonads. In humans, sexual differentiation starts approximately 6
weeks after conception. The end-products of the germ cell cycle are the egg or
Germ cells are specified by signals controlled by zygotic genes. In mammals, a
few cells of the early embryo are induced by signals of neighboring cells to
become primordial germ cells. Mammalian eggs are somewhat symmetrical and
after the first divisions of the fertilized egg, the produced cells are all totipotent.
This means that they can differentiate in any cell type in the body and thus germ
cells. Specification of primordial germ cells in the laboratory mouse is initiated
by high levels of Bone Morphogenetic Protein (BMP) signaling, which activates
expression of the transcription factors Blimp-1/Prdm1 and Prdm14.
Primordial germ cells, germ cells that still have to reach the gonads, also known
as PGCs, precursor germ cells or gonocytes, divide repeatedly on their migratory
route through the gut and into the developing gonads. Female vs. Male Meiosis:
In females, meiosis occurs in precursor cells known as oogonia. Each oogonia
that initiates meiosis will divide twice to form a single oocyte and three polar
bodies. However, before these divisions occur, these cells stop at the diplotene
stage of meiosis I and lay dormant within a protective shell of somatic cells called
the follicle. Follicles begin growth at a steady pace in a process known as
folliculogenesis, and a small number enter the menstrual cycle. Menstruated
oocytes continue meiosis I and arrest at meiosis II until fertilization. The process
of meiosis in females occurs during oogenesis, and differs from the typical
meiosis in that it features a long period of meiotic arrest known as the Dictyate
stage and lacks the assistance of centrosomes.
In males, meiosis occurs in precursor cells known as spermatogonia that divide
twice to become sperm. These cells continuously divide without arrest in the
seminiferous tubules of the testicles. Sperm is produced at a steady pace. The
process of meiosis in males occurs during spermatogenesis.
This principle states that the alleles for a trait separate when gametes are
formed. These allele pairs are then randomly united at fertilization.
Random fertilization is a term used in biology. It refers to the fact that any egg
can be fertilized by any sperm. If you have 8 million eggs and 8 million sperm,
this would mean that each person is one out of 64 million possible combinations.
A uniflagellar sperm cell that is motile is referred to as a spermatozoon, whereas
a non-motile sperm cell is referred to as a spermatium. Sperm cells cannot divide
and have a limited life span, but after fusion with egg cells during fertilization, a
new organism begins developing, starting as a totipotent zygote. The human
sperm cell is haploid, so that its 23 chromosomes can join the 23 chromosomes
of the female egg to form a diploid cell. In mammals, sperm develops in the
testicles and is released from the penis.
The mammalian sperm cell consists of a head, a midpiece and a tail. The head
contains the nucleus with densely coiled chromatin fibres, surrounded anteriorly
by an acrosome, which contains enzymes used for penetrating the female egg.
The midpiece has a central filamentous core with many mitochondria spiralled
around it, used for ATP production for the journey through the female cervix,
uterus and uterine tubes. The tail or "flagellum" executes the lashing movements
that propel the spermatocyte.
During fertilization, the sperm provides three essential parts to the oocyte:
(1) a signalling or activating factor, which causes the metabolically dormant
oocyte to activate;
(2) the haploid paternal genome;
(3) the centrosome, which is responsible for maintaining the microtubule system. Spermatogenesis:
Spermatogenesis is the process by which male primordial germ cells called
spermatogonia undergo meiosis, and produce a number of cells termed
spermatozoa. The initial cells in this pathway are called primary spermatocytes.
The primary spermatocyte divides into two secondary spermatocytes; each
secondary spermatocyte then divides into two spermatids. These develop into
mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte
gives rise to two cells, the secondary spermatocytes, and the two secondary
spermatocytes by their subdivision produce four spermatozoa.
Spermatogenesis takes place within several structures of the male reproductive
system. The initial stages occur within the testes and progress to the epididymis
where the developing gametes mature and are stored until ejaculation. The
seminiferous tubules of the testes are the starting point for the process, where
stem cells adjacent to the inner tubule wall divide in a centripetal direction—
beginning at the walls and proceeding into the innermost part, or lumen—to
produce immature sperm. Maturation occurs in the epididymis.
The process of spermatogenesis is highly sensitive to fluctuations in the
environment, particularly hormones and temperatur